METEOROLOGY 


METEOROLOGY 


WEATHER,  AND  METHODS  OF  FORECASTING 


DESCRIPTION  OF 

METEOROLOGICAL  INSTRUMENTS 


AND 


RIVER  FLOOD  PREDICTIONS  IN  THE  UNITED  STATES 


BY 

THOMAS    RUSSELL 

U.   S.  ASSISTANT  ENGINEER 


THE   MACMILLAN   COMPANY 

LONDON:  MACMILLAN  &  CO.,  LTD. 
1906 

All  rights  reserved 


COPYRIGHT,  1895, 
BY  MACMILLAN  &  CO. 

Set  up  and  electrotyped.    Published  January,  1895.     Reprinted 
May,  1906. 


Norfaooti 

J.  8.  Gushing  &  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.8.A. 


PREFACE. 


IN  recent  years  there  has  been  a  great  development  of  interest  in 
scientific  weather  observation  and  prediction.  Government  weather 
services  have  been  founded  in  most  countries  over  the  world,  and 
weather-maps  are  published  daily  showing  the  weather  over  vast  areas 
of  country  as  reported  by  telegraph.  The  hopes  that  were  once  enter- 
tained that  a  precise  knowledge  of  coming  weather  could  be  gained 
from  the  weather-map  has  not  been  fully  realized.  Cases  are  compara- 
tively rare  where  it  can  be  of  use  in  predicting  the  weather.  There  are 
not  more  than  six  to  twelve  occasions  in  the  course  of  a  year  for  any 
part  of  the  country  where  successful  predictions  can  be  made,  and  for 
some  places  successful  predictions  are  never  possible. 

The  main  object  of  this  book  is  to  explain  the  use  of  the  weather- 
map,  where  it  can  be  of  use,  for  the  purpose  of  making  predictions. 
The  kinds  of  weather  that  can  be  foretold  are  the  great  changes,  and 
these  are  the  ones  most  interesting  to  know.  Successful  continuous 
predictions  for  every  day  are  not  possible.  A  fall  of  temperature  as 
great  as  40  degrees  can  be  foreseen  to  a  certainty  for  most  parts  of  the 
country  east  of  the  Mississippi  River.  The  north-east  rain  storms  along 
the  Atlantic  coast  can  be  successfully  predicted  in  most  cases.  Floods 
along  the  lower  Ohio  and  Mississippi  rivers  can  be  foreseen  from  one  to 
three  weeks  in  advance  of  their  occurrence,  and  the  height  the  water  will 
reach  can  be  assigned  within  a  foot  or  two. 

Rain  occurs,  as  a  rule,  with  the  areas  of  low  air  pressure  that  cross 
the  country  from  west  to  east  and  from  south-west  to  north-east.  The 
average  direction  and  rate  of  motion  of  these  areas  are  known,  but  they 
are  subject  to  many  irregularities.  Rain  can  in  most  cases  be  inferred 
for  regions  over  which  the  areas  are  likely  to  pass.  At  times,  however, 

vii 

266892 


viii  PREFACE. 

great  downpours  of  rain  and  high  winds  occur  out  of  all  proportion  to 
any  condition  of  humidity  of  the  air  or  pressure  gradients  that  can 
be  previously  traced  on  the  weather-maps.  Persons  interested  in  the 
weather,  and  in  a  position  to  examine  the  weather-maps  from  day  to  day, 
would  do  well  to  learn  the  methods  of  making  predictions,  and  be  able 
to  draw  conclusions  in  regard  to  the  coming  weather  and  to  determine 
to  what  extent  such  conclusions  are  trustworthy.  It  is  the  aim  of  this 
book  to  be  of  use  to  such  persons,  and  show  in  what  cases  useful  fore- 
casts of  weather  are  possible.  The  method  is  based  mainly  on  statistics 
of  the  observed  condition  of  the  air  as  to  pressure,  temperature,  and 
humidity  of  particular  types  and  the  weather  following  twenty-four 
hours  or  more  after  the  occurrence  of  the  type. 

A  short  account  of  floods  is  here  given,  and  the  methods  of  predicting 
river  heights  for  some  points  along  the  lower  Mississippi  River  and  its 
tributaries.  The  various  forms  of  meteorological  instruments  are  de- 
scribed with  reference  to  the  principles  involved  in  their  construction. 
A  general  view  is  here  taken  of  all  the  knowledge  relating  to  the  air 
commonly  known  as  the  science  of  meteorology.  Climatology,  or  the 
treatment  of  weather  statistically  by  average  values,  is  only  treated  of 
in  its  broad,  general  features.  Almost  everything  that  is  considered  to 
be  of  interest  in  relation  to  the  weather  is  here  given.  The  principal 
weather  changes  are  described  as  they  occur  in  various  parts  of  the 
world  in  different  seasons  on  land  and  sea,  and  their  causes  narrated  as 
far  as  known.  A  collection  of  facts  is  given  useful  in  forming  a  con- 
ception of  the  phenomena  of  the  atmosphere  as  a  whole,  so  as  to  enable 
those  with  little  time  for  consulting  a  multitude  of  books  to  form  some 
notion  of  the  science  of  meteorology  as  it  is  at  present. 

The  thanks  of  the  author  are  due  to  Professor  Cleveland  Abbe  of 
the  Weather  Bureau,  Washington  Office,  for  kindly  assistance  in  the 
preparation  of  the  work,  and  to  Queen  &  Co.  of  Philadelphia  for  cuts 

furnished. 

T.  R. 


CONTENTS. 


INTRODUCTION. 

PAGE 

The  moon  and  the  weather •        .        .  xvii 

Sun-spots  and  the  weather   .                 xix 

Weather  periodicity xxii 

CHAPTER   I. 

THE   AIR. 

Meteorology,  Air  pressure I 

Properties  of  air 2 

Air  constituents 3 

Insolation 5 

Atmospheric  circulation 8 

Ocean  currents 10 

CHAPTER   II. 

METEOROLOGICAL  INSTRUMENTS. 

Thermometer 15 

Maximum  thermometer 19 

Minimum  thermometer         ...........  20 

Solar  radiation  thermometer         .         .         .         .         .         .         .         .        .         .21 

Radiometer 25 

Actinometer 26 

Barometer 28 

Aneroid  barometer 33 

Hypsometer          .............  34 

Dew-point  apparatus     ............  36 

Psychrometer       .............  37 

Hair  hygrometer 38 

Rain  gauge 39 

Snow  gauge 39 

Percolation  gauge 40 

Anemometer 40 

ix 


X  CONTENTS. 

PAGE 

Beaufort  scale      •        •••*«.......      42 

Wind  vane  .         .        •        .        .         .         .         ..        .         .        .        .         .43 

Self-register          .         .        ...        .        .        ..,       4         .         .         .         '43 

Anemograph,  Pluviograph    .         . 44 

Nephoscope 45 

Sunshine  recorder        ............      46 

Evaporometer      .         .        .         .         .         .  .  .        .        .        .47 

Electrometer        .         .         . .        .        .        .48 

River  gauge          .        .  .        .         .  .        .  -       .        .'       .        .49 

Current  meter .        .        ;        .        .      51 

CHAPTER   III. 

TEMPERATURE   AND   PRESSURE. 

Air  temperature -52 

Daily  range  of  temperature •  .                 .        .  .      54 

Temperature  range  at  sea 55 

Mean  temperatures       .         . .56 

Ocean  temperature .  -57 

Earth  temperature .        .        .  58 

Maximum  temperatures         .         .         .         .         .         .         .        .        ^        .  59 

Minimum  temperatures         .         .         .         .         .         .         .        .        ...  .59 

Temperature  variability         ...........      60 

Dew,  Frost .        .  .      61 

Lake  climate 63 

Temperature  and  height .  •  .      63 

Pressure  of  air ...      64 

Diurnal  oscillation 64 

Variation  with  latitude .        .        *  -65 

Effect  of  moon  on  pressure    .     ,    .         .       *.        •.        .        .                 «  .      65 

Monthly  range       .         .         .         .         .         .         .         .         .       ' .        .  .67 

Annual  range         .         .         .         .         ....               •,  .         .  .67 

Distribution  of  pressure         .         .         .         .        .         .         .         .         .  .       67 

January  pressure    .         .         .         .         .         .        .         .         .         .        *  .       68 

July  pressure          .         .        .        .        .        .        •        ......  .      69 

CHAPTER   IV. 

EVAPORATION,   CLOUDS,    RAIN,   AND   SNOW. 

Daily  change  of  vapour  pressure  .        .        .        .        .        .        ...',.  .  71 

Yearly  change  of  pressure     .         .         .  <      .         .         .         .         .         .  .  71 

Cloud  formation   .         .        .       '.       '.        .         .         .       '.         .         .  .  .  72 

Cloud  classification      '.         .         .     *   .-      .         .        .         .        .         .  .  .  72 

High  clouds :  Cirrus,  Cirro-stratus        .        .         .         .         .         .        «  .  .  73 

Intermediate  clouds :  Cirro-cumulus,  Cumulo-cirrus,  Strato-cirrus          .  .  •  74 

Lower  clouds  :  Strato-cumulus,  Nimbus-cumulus,  Cumulo-nimbus,  Stratus  75 


CONTENTS.  xi 


Cloud  shadows,  Cloud  variation 76 

Fog  formation 78 

Atmospheric  electricity 81 

Rainfall        ....  .  .83 

Causes  of  rain       .                  85 

Distribution  of  rainfall 86 

Dry  regions 88 

Rainless  regions .         .         .         .  89 

Trade-wind  rains ,    .       -V   .'.  •         »         •         •         •  89 

Rainfall  in  United  States      .         ,         .         .         v      .*        V   'V,       ?*        .  90 

Snowfall       .         .         .         .         .         «         .         .  ••'.'"    «         «t       •         «    ' '    *         •  95 

Glaciers        .         .         .         .         .        *         ,         <         .         *        .       '•     ".••         •  97 

Sleet,  Hail    .                           ,                                    .                                                      .  98 

Constancy  of  climate    .         .         ,'        .         .         .     '••«''.';  •         .         ..-    ..         .  100 

Constancy  of  rainfall    .         .         .         .         .         ,        .  .       .         i'        ,         .         .  101 

Constancy  of  temperature     .        .        .        ...  ,v    -.       ^»        ;.        .         .  103 

CHAPTER  V. 

WINDS,  THUNDERSTORMS,  AND  TORNADOES. 

Wind  velocity      .         .        ..     • '/*-        t.       .         »         .         .         .         .         .         .  105 

Diurnal  range  of  wind  velocity      .         .         .         i         .         ....         .         .  106 

Daily  variation  of  wind  direction  .         .         .         .         .         .         .         i        ..         .  107 

Land  and  sea  breeze     .         ....         .         .         .         .         .         *  ;.     .  107 

Monsoon _.         ....  109 

Mountain  winds   .         .  ' ,   t     .         .         .  1 1 1 

Fohn  wind 113 

Chinook,  Mistral,  Bora,  Tramontana,  Gregale,  Buran 114 

Purga,   Blizzard,   Northers,   Barber,   Pamperos,    Southerly   Buster,    Nor'wester, 

Scirocco     .         .         .         .         .         .         .         .         .         .         .         .         .115 

La  Veche,  Leste,  Desert  winds,  Harmattan  .         .         .         .         .         .         .         .116 

Khamsin,  Simoon,  Wind  roses 117 

Thunderstorms     .       ^       . 118 

Isobronts      .         •    .     *         •         '••        .x 119 

Lightning,  Ball  lightning,  Fulgurites     .         .         .       ;.         .         t  '      . .  .;  •- ...         .  120 

Periodicity  of  thunderstorms       •*         .,        .         .•        .         .         i'     .-  T'  *  ..      .  121 

Electrical  storms           .         .         .       -,         .         »,        .         ^        .      '  .       ..         .  122 

Cause  of  lightning        .         .         .         .        ,.         .         .        v        .         .  '  >    .         .  122 

Cause  of  thunderstorms        .         .         .         .         . 125 

Tornado 126 

CHAPTER   VI. 

OPTICAL   APPEARANCES. 

Flattening  of  sun,  Looming  up      ..........  131 

Mirage,  Dispersion,  Rainbows      ...........  132 


Xll  CONTENTS. 


Secondary  bow,  Wind  galls,  Fog  bows,  Brocken  spectre,  Glories,  Corona     .         -133 

Aureole,  Halos,  Parhelia 134 

Anthelion,  Luminous  cross,  Blue  sky,  Red  sunsets,  Twilight         .         .         .         .     135 

Ice-blink,  Snow-banners,  Aurora 136 

Magnetic  elements 138 

CHAPTER   VII. 

WEATHER-MAPS. 

Isobars,  Isotherms,  Isohyetals,  Lows 141 

Highs,  Weather-map,  Winds  in  lows 142 

Direction  of  winds  around  low  pressure         .         .- 144 

Pressure  gradient 145 

Cyclones 147 

Movement  of  cyclone 148 

Cyclone  paths 151 

Average  wind  velocity  in  lows      .         .         .         .         ...         .       . ."        .154 

Greatest  wind  velocity  in  lows      .         .         .         .         .         .         ..         .         •     !55 

Table  I.,  low-pressure  areas  with  rise  and  fall       .         .         .         .         .         .         .156 

Table  II.,  low-pressure  areas  and  direction  of  motion .157 

Table  III.,  low-pressure  areas  and  depth  of  depression 158 

Table  IV.,  low-pressure  areas  and  change  of  pressure  at  centre     .         .         .         -159 
Table  V.,  motion  of  low  pressure  in  relation  to  high  pressure       .         .         .         -159 

Conclusions  in  regard  to  motion  of  lows 160 

High  pressures     .     .    .         .         .         .         .         . 161 

Tropical  cyclones          .  .... 165 

Management  of  ship  in  cyclone    .       ..       ..         .         .         .         .         .         .         .     173 

CHAPTER  VIII. 

WEATHER   PREDICTIONS. 

Basis  of  predictions 178 

Estimate  of  motion  of  low    .         .         .         .         .         .         .         .         .         .         .179 

Storm  wind  and  gradients  of  pressure 181 

Winds  and  pressure  fall 181 

Rain  and  low  pressure 181 

Quantity  of  rain 182 

Rain  and  high  pressure 182 

Secondary  low  pressure .184 

Remarks  on  rain  predictions 1 84 

Weather-flags 186 

Spectroscope  and  rain 189 

Cold  waves  .         .         .         .         .         •         •         •         •         •         •         •         •         .189 

Number  and  area  of  cold  waves 190 

Intensity  of  cold  waves      "   . 191 


CONTENTS.  xiii 


Fall  of  temperature -and  gradient 192 

Place  of  greatest  fall 192 

Frost  predictions 194 

Long-time  predictions 195 

CHAPTER   IX. 

RIVERS  AND  FLOODS. 

Kinds  of  floods 198 

Rainfall  floods 200 

River  courses 202 

Different  classes  of  flood  streams 204 

Flood  combinations      ............  209 

Effects  of  forests 210 

River  records 210 

Predictions  of  stages  without  gauges 211 

Rainfall  and  river  rise 212 

River  discharge 215 

CHAPTER  X. 

RIVER-STAGE   PREDICTIONS. 

Methods  of  predicting  river  stages 219 

Floods  at  Paris,  France 221 

River-stage  predictions  for  Pittsburg,  Pa 222 

Wheeling,  W.Va. ;  Parkersburg,  W.Va. ;  Marietta,  Ohio ;  Cincinnati,  Ohio  .  225 

Louisville,  Ky. ;  Evansville,  Ind. ;  Mount  Vernon,  Ind 233 

Cairo,  111 233 

Carthage,  Tenn. ;  Nashville,  Tenn. ;  Eddy ville,  Ky.      .         .         .         .         .  238 

Chattanooga,  Tenn. ;  Decatur,  Ala. ;  Florence,  Ala. ;  Johnsonville,  Tenn.    .  240 

Davenport,  la. ;  Omaha,  Neb. ;  Kansas  City,  Mo. ;  Jefferson  City,  Mo.         .  241 

St.  Louis,  Mo 243 

Memphis,  Tenn. ;  Helena,  Ark. ;  Arkansas  City,  Ark. ;  Greenville,  Miss.     .  261 

Vicksburg,  Miss.   .         .         .         .         .         .         .         .         .         .         .         .  262 

Mississippi  River,  comparative  high-water  stages  ......  266 


ILLUSTRATIONS. 


FIGURE  PAGB 

1.  Bacteria        .        . -.'...        .  .        4' 

2.  Low-temperature  apparatus  .         .         .         .        '.      •  .         .         .         .  .       18 

3.  Solar  radiation  thermometer          .         .         .         *         •         •        .         «  .21 

4.  Radiometer.         .         .       ...         .         .         .         .         .         *  ,  .  .       25 

5.  Barometer .         .         .         ,         .  .       28 

6,7,8.     Verniers        .         .         .         .         .      '.--...         .         .        .         .  .       30 

9.     Aneroid  barometer       .         .         .         .      .  .         .         .         .         .         •  •       33 

10.  Rain  gauge  .         .         .         *         ....         »         .         .         .  39 

1 1 .  Anemometer         .        .         .         .  .         .        .         .        .         .  41 

12.  Self-register          .         .        '.        .        . 44 

13.  Nephoscope         .      ".        .        .        .        .        .        .      -  .'-       ....      45 

14.  Sunshine  recorder         .         .         .         .•  •         •         .         .        t.  .       46 

15.  Evaporometer      .        .        .        .x       ,        .        .        .        .       • ..       .  .      47 

16.  Electrometer        .         .         .         .         .         .         .         .         .         .         .  .48 

17.  Current  meter      .         „'        .         .         .         .       ..         ...         .  .       51 

1 8.  Trace,  showing  range  of  atmospheric  electricity    .         .         .         -         .  .       82 

19.  Diagram,  mountain  winds     .         ...         .         .         *        .         .         .  .     m 

20.  Section  showing  thunderstorm      .         .         .         .         .         .         .         .  .125 

21.  Tornado       .         .         .        -.         .         .         .         .         .         ...  .     127 

22.  Winds  around  low  pressure  .         .         ...         ...         .         .  .     143 

23.  Cyclone  paths  in  United  States    .         .         .  •         •          .        .  .     152 

24.  Map,  average  wind  velocity  in  low  pressures          .         .         .         .         .  .     154 

25.  Map,  greatest  wind  velocity  in  low  pressures         .        «         .         .         .  .     155 

26.  Winds  in  high  pressure         .         .         .         .         ...._..     161 

27.  Path  o         it  India  cyclone .         .         .         .         .         .         .        ...  .168 

28.  Wind  a         ons  around  hurricanes        .         .         ....        r.         .  .     170 

29.  Secondary  depression  .        »         .         .         .     '    .         ...         .  .184 

Weather-flags    .  .         ...         .         .         .         .         .         .         .  .187 

Cold-wave  weather-map        .         .         .         .         .         .         .         .         .        «  .190 

Map,  drainage  area  above  Pittsburg,  Pa.     ", :''••••         •         •  *        •  •     223 

Map,  drainage  area  above  Cincinnati,  Ohio  .         .       ^        «         .         •         .  .     227 

Map,  drainage  area  above  Cairo,  111.     .         .         ....         .         .  .     232 

Map,  drainage  area  above  St.  Louis,  Mo.      .         .         .         .        *        .         .  .     244 

Map,  overflow  region  of  lower  Mississippi  River  .......     260 


INTRODUCTION. 


THE  earliest  ideas  of  the  weather  were  that  the  stars  and  planets  had 
some  influence  on  it.  Astrological  meteorology  began  to  decline  with  the 
invention  of  the  thermometer  and  barometer,  and  with  the  discovery  of 
the  true  theory  of  the  solar  system  by  Copernicus,  which  explains  the 
motion  of  the  planets  in  accordance  with  simple  physical  laws.  The 
emptiness  of  astrology  soon  began  to  be  perceived.  The  belief  that  the 
moon  has  some  influence  on  the  weather  has  survived  and  at  the  present 
time  prevails  to  some  extent.  The  changes  of  the  moon,  more  espe- 
cially, have  been  supposed  to  influence  the  weather.  There  has  been  a 
great  deal  of  statistical  research  attempting  to  show  a  connection  be- 
tween the  moon  and  the  weather,  and  it  is  worthy  of  some  attention. 
All  such  attempts,  however,  to  connect  the  moon  and  the  weather  have 
signally  failed.  Newton's  discovery  of  the  law  of  gravitation,  and  that 
the  attraction  of  the  moon  was  the  cause  of  the  tides,  strengthened  the 
belief  for  a  time  that  the  moon  must  have  an  influence  on  the  weather. 
It  opened  up  a  wide  field  of  investigation,  and  a  great  deal  has  been 
done  in  comparing  the  weather  with  the  motion  of  the  moon  and  its 
changes  of  phase. 

It  was  a  matter  of  observation  from  the  earliest  times  that  the  tides 
followed  the  moon.  The  real  cause  was  a  mystery  until  Newton's  dis- 
covery. The  speculations  as  to  the  cause  of  the  tides  were  various. 
Galileo  deplored  the  superstitious  tendency  of  his  time  in  attributing 
the  tides  to  any  action  the  moon  could  have.  His  theory  was,  that  the 
tides  were  due  to  the  double  motion  of  the  earth  on  its  axis  and  around 
the  sun,  and  the  water  of  the  ocean  not  following  the  change  in  the 
direction  of  motion  as  quickly  as  the  solid  parts  of  the  earth. 

The  moon  must  cause  a  tide  in  the  air  as  well  as  the  ocean,  and  it 
seems  reasonable  that  it  might  have  some  influence  on  the  weather, 


xviii  INTRODUCTION. 

though  very  small.  The  attitude  of  scientifically  qualified  persons 
towards  the  belief  in  the  moon  as  affecting  the  weather  has  been  one 
of  disapproval  or  open  contempt.  This  is  not  grounded  on  sufficient 
examination  in  most  cases,  but  is  largely  the  result  of  prejudice,  pos- 
sibly arising  from  the  immense  amount  of  charlatanism  in  connection 
with  the  moon  and  the  weather  of  which  the  world  has  been  the  victim. 

Weather  records  for  a  number  of  places  that  have  been  examined 
show  that  there  is  a  somewhat  greater  tendency  to  rain  in  the  quarter 
after  full  moon  than  at  other  times ;  there  is  a  greater  probability  of 
rain  with  the  moon  in  perigee  than  apogee.  There  is  no  doubt  in  re- 
gard to  this  relation  of  the  moon  and  rain,  but  the  difference  is  not 
known  even  approximately,  and  it  varies  for  different  parts  of  the  earth. 
It  is  of  no  practical  value  for  weather  prediction.  Attempts  have  been 
made  to  show  that  there  is  less  cloudiness  at  full  moon  than  at  other 
times.  The  results  are  contradictory  for  different  places.  The  effect 
is  either  so  small  that  the  longest  series  of  records  does  not  show  it,  or 
it  does  not  exist.  The  effect  of  the  moon  on  the  frequency  of  thunder- 
storms is  inappreciable.  As  regards  the  winds,  northerly  winds  are 
more  prevalent  during  the  last  quarter  of  the  moon,  and  southerly  winds 
more  frequent  during  the  first  quarter  than  at  other  times.  No  conclu- 
sion can  be  drawn  in  regard  to  the  varying  force  of  the  wind  for  the 
different  quarters  of  the  moon.  With  regard  to  change  of  the  weather 
and  change  of  the  moon  in  five  thousand  cases  examined,  eighteen 
hundred  showed  a  change  of  weather  and  thirty-two  hundred  no  change. 

The  effect  of  the  moon  on  the  pressure  of  the  atmosphere  is  very 
slight.  There  is  an  ebb  and  flow  that  is  only  perceptible  near  the 
equator  by  the  most  refined  instrumental  means  of  observation.  The 
difference  between  the  least  and  greatest  pressure  due  to  the  effect  of 
the  moon  is  only  0.004  of  an  inch.  The  pressure  of  the  air  is  probably 
greater  with  the  moon  farthest  from  the  earth,  and  greater  at  quadra- 
tures than  syzygies.  No  predictions  of  weather  of  any  value  can  be 
made  based  on  pressure  variations  due  to  the  moon. 

Some  heat  is  received  at  the  surface  of  the  earth  from  the  moon,  and 
this  being  the  case,  some  must  be  absorbed  by  the  air.  The  planets,  or 
brightest  fixed  stars,  give  no  appreciable  heat  that  can  be  detected  with 
the  most  refined  means  of  measurement.  This  is  not  conclusive,  how- 


INTRODUCTION.  XIX 

ever,  as  the  heat  may  be  entirely  absorbed  by  the  air.  It  was  a  very 
pretty  speculation  at  one  time  that  the  greater  warmth  of  the  northern 
hemisphere  as  compared  with  the  southern  was  due  to  the  radiation 
'from  the  greater  number  of  stars  visible  in  the  northern  heavens. 
More  extensive  observations  of  temperature  show  that  there  is  probably 
no  difference  between  the  two  hemispheres.  Comparative  statistics  for 
one  hundred  and  fifty-three  years,  some  of  them  comet  years,  show  that 
comets  have  no  effect  on  the  temperature  or  any  other  element  of  the 
weather. 

The  vast  number  of  meteors  moving  about  the  sun,  some  of  which 
pass  through  the  air  leaving  a  temporary  trail  of  light,  and  then  passing 
on  or  sometimes  falling  to  the  earth,  probably  have  no  effect  on  the 
temperature  of  the  air  or  the  weather.  There  are  two  principal 
groups  of  meteors  moving  about  the  sun.  It  was  at  one  time  surmised 
that  these  coming  between  the  earth  and  the  sun,  which  occurs 
February  /th  to  I2th,  and  about  May  nth,  might  cut  off  the  heat  of 
the  sun  enough  to  cause  the  peculiar  anomaly  of  temperature  observable 
about  those  times,  especially  in  May.  It  is  not,  however,  the  cause. 
The  phenomenon  of  the  three  cold  days  in  May,  the  loth,  nth,  and 
1 2th,  is  attested  by  the  eighty-six  years  of  weather  record  at  Berlin,  and 
is  explainable  by  the  general  circulation  of  the  air  and  the  sequence  of 
high  and  low  pressure  areas. 

The  attempts  to  prove  a  connection  between  sun-spots  and  the 
weather  are  of  some  interest.  The  subject  once  received  a  good  deal 
of  attention.  As  the  sun  is  the  prime  cause  of  all  changes  in  the  air, 
any  changes  going  on  on  the  surface  of  the  sun  might  have  an  influence 
on  the  weather.  It  is  extremely  doubtful,  though,  that  there  is  any  con- 
nection. 

When  the  sun  is  examined  with  coloured  glass,  dark  spots  of  various 
form  and  size  are  perceived  singly  and  in  groups.  They  are  mostly 
seen  in  the  latitude  of  10  to  15  degrees  on  the  sun,  and  not  in  the 
vicinity  of  its  equator  or  poles.  They  appear  first  on  the  eastern  edge 
of  the  sun,  and  moving  west  disappear  in  13  days,  to  reappear  after  a 
similar  period.  Their  apparent  motion  is  due  to  the  rotation  of  the  sun 
on  its  axis,  the  period  of  which  is  25.78  days.  These  spots  are  of  great 
extent,  occupying  at  times  hundreds  of  thousands  of  square  miles.  The 


XX  INTRODUCTION. 

fact  that  they  appear  only  in  certain  zones  indicates  that  the  surface  of 
the  sun  is  not  physically  the  same  everywhere.  The  spots  seem  to  be 
associated  in  some  way  with  cyclonic  motions  of  the  gases  on  the  sur- 
face of  the  sun.  They  have  a  proper  motion  relative  to  a  point  on  the 
surface  of  the  sun,  as  shown  by  the  fact  that  different  spots,  especially 
in  different  latitudes,  give  slightly  different  times  for  the  sun's  period  of 
rotation.  The  spots  are  related  in  some  way  to  the  red  protuberances 
seen  at  the  edge  of  the  sun,  which  are  sudden  bursts  of  heated  gases 
from  the  surface  of  the  sun  into  the  higher  regions  of  its  atmosphere. 
From  the  known  approximate  height  of  these  protuberances,  and  the 
law  of  the  dynamical  cooling  of  gases  with  ascent,  the  temperature  of 
the  sun's  surface  is  estimated  to  be  at  least  as  high  as  12,000°  F. 

There  is  a  period  observable  in  the  frequency  of  the  sun-spots. 
There  is  an  increase  in  their  number,  and  then  a  decrease.  The  period 
is  slightly  variable  ;  on  the  average  it  is  n.i  years.  The  interval  from 
the  time  of  least  to  greatest  frequency  is  3.7  years ;  the  time  from  the 
greatest  to  least  is  twice  as  long,  or  7.4  years.  The  heat  of  the  surface 
of  a  sun-spot  is  less  than  that  from  the  bright  surface  of  the  sun  as 
indicated  by  a  thermopile.  Among  the  first  attempts  to  trace  a  connec- 
tion between  sun-spots  and  the  occurrences  on  the  earth  was  Herschel's 
notable  investigation  into  the  relation  of  the  price  of  wheat  and  the  fre- 
quency of  sun-spots  in  different  years.  It  was  found  that  for  Great 
Britain  the  years  of  lowest  price  corresponded  with  the  greatest  fre- 
quency of  sun-spots,  the  inference  being  that  years  of  great  frequency 
are  favourable  to  the  growth  of  wheat,  and  on  account  of  the  larger  yield 
the  price  is  lower.  The  same  was  found  to  be  true  for  prices  on  the 
Continent.  Though  this  was  true  for  the  eighteenth  century,  it  has  not 
been  true  for  the  nineteenth.  Herschel's  inquiry  was  made  before  the 
periodicity  in  sun-spot  frequency  had  been  detected.  The  statistical 
examination  of  many  things  has  been  made  in  relation  to  sun-spot 
frequency,  air-temperature,  pressure,  cloudiness,  rainfall,  auroras,  and 
cyclones.  The  result  for  the  temperature  of  the  air  in  relation  to  the 
sun-spot  frequency  is  very  doubtful.  There  is  apparently  a  relation 
between  pressure  of  the  air  and  sun-spots  for  southern  Asia  at  least. 
In  the  years  of  greatest  sun-spot  frequency  the  pressure  is  slightly 
higher  than  in  years  of  least  frequency.  Cyclones  were  found  by 


INTRODUCTION.  xxi 

Meldrum  to  be  most  frequent  in  the  south  Pacific  Ocean  in  sun-spot 
years. 

If  sun-spots  influence  radiation  from  the  sun,  it  is  natural  to  suppose 
they  must  have  an  influence  on  the  rainfall  over  the  earth  through  its 
effect  on  air  currents.  This  is  the  more  reasonable  if  cyclones  which 
are  always  accompanied  by  rain  are  more  frequent  in  years  of  great  sun- 
spot  frequency.  There  is  greater  difficulty  in  comparing  rainfall  with 
sun-spot  frequency  than  any  other  meteorological  element,  as  the  annual 
rainfall  varies  greatly  in  places  quite  close  together.  One  set  of  stations 
shows  greater  rainfall  in  sun-spot  years,  while  another  set  shows  less. 
The  total  rainfall  of  India  shows  no  evidence  of  an  eleven-year  period. 
Stations  on  the  sea-coast,  where  the  rainfall  is  fully  dependent  on  the 
wind,  favour  the  view  that  sun-spots  increase  the  rainfall.  Inland  sta- 
tions, where  the  wind  varies  from  local  causes,  are  unfavourable.  The 
year  1867,  which  was  one  of  least  sun-spot  frequency,  was  rainy  every- 
where. That  there  is  any  dependence  of  rainfall  on  sun-spot  frequency 
is  very  doubtful.  If  there  is  a  period  in  rainfall  corresponding  to  sun- 
spots,  it  should  be  reflected  in  the  heights  of  rivers.  In  years  of  great- 
est sun-spot  frequency  more  water  flows  through  the  rivers  of  Europe 
than  in  other  years.  In  general,  the  years  in  which  the  high  waters  of 
the  Nile  are  below  the  average  are  near  the  years  of  least  sun-spot  fre- 
quency. The  same  is  true  of  the  level  of  the  great  lakes  in  the  United 
States  and  Canada.  In  the  outflow  of  the  Mississippi  River  there  is  no 
trace  of  an  eleven-year  period,  nor  does  the  high  water  have  any  relation 
to  the  years  of  greatest  sun-spot  frequency.  The  frequency  of  thunder- 
storms shows  no  relation  to  sun-spots.  Hail-storms,  it  is  claimed,  are 
more  frequent  in  years  of  greatest  sun-spot  frequency  than  in  others. 

On  the  whole,  the  weight  of  evidence  seems  to  favour  the  view  that 
there  is  no  connection  between  sun-spots  and  meteorological  phenomena, 
or  at  least  the  connection  is  a  very  slight  or  a  very  remote  one.  There 
does  seem  to  be  a  period  in  the  frequency  of  thunderstorms  dependent 
on  the  time  of  rotation  of  the  sun  on  its  axis,  26  days,  and  likewise  a 
period  in  the  variation  of  the  earth's  magnetic  elements  depending  on 
the  rotation  of  the  sun,  but  it  is  doubtful  if  even  these  are  related  in 
any  way  to  the  sun-spots. 

There  has  been  a  great  deal  of  research  to  discover  some  periodicity 


XX11  INTR  OD  UCTION. 

in  the  recurrence  of  weather.  If  such  a  period  could  be  found,  the  pre- 
diction of  weather  would  be  a  very  simple  matter,  as  it  would  only  be 
necessary  to  have  observations  of  the  weather  over  one  of  the  periods. 
No  attempts  to  discover  any  such  periodicity  have  ever  been  attended 
with  the  least  success. 

When  the  barometer  was  invented,  and  the  daily  variation  of  the  air 
pressure  discovered,  it  was  expected  that  with  the  improvement  of  the 
instrument  it  would  be  possible  to  foretell  the  weather.  The  improve- 
ment was  slow  in  realization.  The  most  important  step  in  the  scientific 
study  of  the  weather  was  taken  about  100  years  ago,  with  the  founding 
of  the  Meteorological  Society  of  the  Palatine  in  Germany.  It  was  then 
found  for  the  first  time  that  oscillations  of  air  pressure  occurred  at  the 
same  time  over  large  areas  of  country,  and  that  low  pressures  occur 
earlier  to  the  north  and  west  of  a  place  and  later  to  the  south  and  east 
of  it,  and  in  fact  that  there  is  a  progression  of  the  low  pressure  from 
west  to  east. 

With  the  invention  of  visual  telegraphy  in  1793,  there  was  a  project 
formed  for  the  transmission  of  weather-reports  from  place  to  place,  but 
it  was  never  carried  out.  Since  the  invention  of  the  electric  telegraph, 
government  weather-services  have  been  established  in  countries  all  over 
the  world.  Information  of  the  condition  of  the  air,  its  temperature, 
pressure,  cloudiness,  the  wind  and  rain  for  some  particular  instant  of 
time,  is  telegraphed  from  widely  separated  places  to  some  central  station, 
and  shown  graphically  in  a  generalized  form  by  lines  or  by  shading  on  a 
map  of  the  country.  These  maps,  usually  published  once  a  day  in  large 
cities,  are  known  as  weather-maps.  The  first  of  modern  weather-maps 
was  made  in  the  year  1854.  During  the  Crimean  War,  on  November  14, 
1854,  a  great  storm  occurred  on  the  Black  Sea  which  almost  destroyed 
the  allied  fleet,  and  caused  the  loss  of  the  French  man-of-war  Henry 
IV.  A  storm  was  known  to  have  prevailed  several  days  before  in  west- 
ern Europe.  On  the  request  of  the  French  Minister  of  War  Vaillant, 
the  astronomer  Leverrier  investigated  all  the  circumstances  attending 
the  progress  and  formation  of  the  storm.  Observations  of  pressure,  tem- 
perature, and  wind  were  gathered  from  all  the  observatories  in  Europe 
and  charted.  The  investigation  showed  that  the  storm  moved  from  the 
north-west  to  the  south-east.  With  telegraphic  communication  between 


INTRODUCTION.  Xxiil 

Vienna  and  the  Crimea,  and  weather-reports  from  the  west  of  Europe, 
it  was  seen  that  it  would  have  been  possible  to  warn  the  fleet  in  time  to 
have  prevented  disaster. 

This  event  aroused  very  widespread  interest,  and  stimulated  the 
study  and  observation  of  the  weather.  The  numerous  weather-services 
that  have  been  founded  over  the  world  since  show  the  desire  on  the  part 
of  the  public  for  as  trustworthy  information  in  regard  to  the  weather  as 
it  is  possible  to  obtain. 


CHAPTER   I. 

THE  AIR. 

Meteorology.  —  Meteorology  is  the  study  of  the  air,  all  its  properties, 
motions,  and  appearances.  The  orderly  arrangement  and  statement 
of  all  facts  relating  to  the  atmosphere  and  its  changes,  and  the  assign- 
ment of  their  causes,  is  the  province  of  meteorology. 

The  atmosphere  is  a  body  of  mixed  gases  surrounding  the  earth  and 
extending  to  a  height  of  at  least  fifty  miles  above  the  surface.  The 
component  gases  form  a  mechanical  and  not  a  chemical  mixture.  The 
various  gases  fit  into  the  interstices  of  each  other.  Each  one  tends  to 
form  an  atmosphere  by  itself  as  if  the  others  were  not  present. 

Air  Pressure.  —  Air  exerts  a  pressure  on  everything  in  it  equal  to  the 
weight  of  the  column  of  air  above  it.  The  air  pressure  varies  from 
time  to  time.  At  the  level  of  the  sea,  on  the  average,  the  air  pressure 
is  equal  to  that  exerted  by  a  column  of  mercury  29.92  inches  in  height, 
or  a  column  of  water  about  34  feet  high,  equal  to  about  14.67  pounds 
to  the  square  inch.  The  air  tends  to  expand,  but  is  restrained  by  the 
weight  of  the  air  above  it,  and  the  pressure  of  the  air  around  it.  The 
pressure  diminishes  with  ascent  in  the  air,  there  being  less  above  a 
plane,  the  greater  the  height.  This  property  of  diminishing  pressure 
with  height  is  used  to  determine  heights  above  sea  level.  A  con- 
venient approximate  rule  is  the  following :  The  height  of  a  place  in 
feet  is  equal  to  the  product  of  two  factors,  the  first  a  fraction  equal  to 
the  difference  between  the  pressure  at  the  place  and  the  sea  level 
divided  by  the  sum  of  the  pressures;  and  the  second,  the  number 
55>63O,  when  the  average  temperature  of  the  air  between  the  two 
places  is  60°.  The  number  increases  at  the  rate  of  117  for  every 


2  METEOROLOGY. 

Ar^i>*u*fyAj 

degree  above  60°,  and  diminishes  the  same  amount  for  every  degree 
below  it. 

Properties  of  Air.  —  A  cubic  foot  of  pure  dry  air,  at  a  temperature 
of  32°  and  a  pressure  of  30  inches,  weighs  1.294  ounces.  The  density  of 
air  is  yy^-  that  of  water.  The  whole  body  of  air  forms  the  y^ -Jinro 
part  of  the  mass  of  the  earth.  A  homogeneous  atmosphere  equal  in 
mass  to  the  present  one,  uniform  in  density,  and  of  the  same  density  as 
the  average  at  the  surface  of  the  earth,  would  extend  to  a  height  of 
26,223  feet.  The  duration  of  twilight  would  indicate  that  there  is  no 
appreciable  light  reflected  by  any  atmosphere  beyond  a  height  of  fifty 
miles,  yet  its  free  surface  is  probably  much  higher,  as  shown  by  shoot- 
ing stars,  the  incandescence  being  due  to  friction  of  the  meteors  moving 
through  the  air  at  a  very  great  velocity. 

A  volume  of  air  remaining  the  same,  its  pressure  varies  with  the 
temperature,  being  proportional  to  the  absolute  temperature.  Absolute 
temperature  is  reckoned  from  absolute  zero,  a  point  459°  below  the 
zero  of  the  Fahrenheit  scale.  The  absolute  zero  is  a  fiction  derived 
from  the  kinetic  theory  of  gases.  Air  expands  the  ^T  part  of  its 
volume  for  an  increase  of  one  degree  in  temperature.  If  restrained  in 
volume,  its  pressure  increases  in  the  same  ratio. 

Air  on  being  compressed  is  heated,  and  in  expanding  cools.  Changes 
of  pressure,  and  consequent  change  of  temperature,  without  the  addi- 
tion or  subtraction  of  heat,  is  called  an  adiabatic  process.  For  air  at 
a  pressure  of  30  inches  and  a  temperature  of  60°,  an  increase  of  one 
inch  in  pressure  will  raise  the  temperature  5°.  Doubling  the  pressure 
to  60  inches  will  raise  it  to  175°. 5;  diminishing  the  pressure  one-half 
lowers  the  temperature  to  2°.4. 

Specific  Heat.  —  The  amount  of  heat  required  to  raise  a  given  quantity 
of  air  a  degree  in  temperature,  when  maintained  at  a  constant  pressure, 
is  0.24  of  the  amount  required  to  raise  an  equal  weight  of  water  a 
degree.  This  ratio  is  called  the  specific  heat  of  air.  When  the  volume 
of  air  is  maintained  constant,  the  amount  of  heat  required  to  change  its 
temperature  is  less  than  when  free  to  expand,  in  the  ratio  of  i  to 
1.41.  The  greater  amount  required  when  free  to  expand  is  due  to  the 
expenditure  of  heat  in  doing  the  work  of  pushing  the  pressure  of  the 
surrounding  air  aside.  The  specific  heat  of  vapour  of  water  is  0.48. 


THE  AIR.  3 

Air  cools  in  ascending  at  the  rate  of  0.55  of  a  degree  for  every  100  feet 
of  ascent. 

Air  Constituents. — Air  is  composed  of  the  gases  nitrogen,  oxygen, 
carbon  dioxide  (carbonic  acid),  vapour  of  water,  and  a  slight  amount  of 
ammonia.  In  dry  air  the  gases  are  in  the  proportion  by  volume  of  100 
parts ;  nitrogen,  79.02  ;  oxygen,  20.95;  carbon  dioxide,  0.03.  By  weight, 
the  composition  of  dry  air  is,  nitrogen,  76.78;  oxygen,  23.17;  carbon 
dioxide,  0.05. 

With  the  density  of  dry  air  as  unity,  the  density  of  oxygen  is  1.10563; 
of  nitrogen,  0.97137;  carbon  dioxide,  1.5201  ;  vapour  of  water,  0.622. 

The  percentage  of  oxygen  in  the  air  varies  at  different  times  and  in 
different  places  from  20.47  to  20.96.  When  the  wind  is  from  the  south 
there  is  less  than  when  from  the  north.  The  amount  diminishes  slightly 
with  ascent  in  the  air ;  this  is  due  to  its  greater  density  as  compared 
with  nitrogen.  • 

All  animal  life  and  all  fires  are  supported  by  oxygen.  The  combus- 
tion of  carbon  and  oxygen,  or  their  chemical  combination,  produces 
carbon  dioxide.  Plants  decompose  carbon  dioxide  and  give  off  oxygen 
to  the  air.  A  grown  person  consumes  about  420  gallons  of  oxygen  in  a 
day.  All  the  fires  on  the  earth  burn  in  a  century  about  as  much  as  is 
contained  in  the  air  over  a  square  degree  on  the  earth's  surface,  equal 
to  70  miles  on  a  side.  The  principal  source  of  carbon  dioxide  is  the 
sea.  One  litre  of  sea- water  contains  98.3  milligrammes.  One  decimetre 
square  of  green  leaves  decomposes  in  an  hour  seven  cubic  centimetres 
of  carbon  dioxide  in  the  sun,  and  three  in  the  shade. 

The  invisible  moisture  contained  in  the  air  in  the  form  of  vapour 
forms  a  very  variable  part  of  the  air  at  different  times.  It  is  no  more 
than  y-^g-g-  part  on  a  cold  winter  day,  and  on  a  warm  summer  day  may 
be  as  much  as  -fa  by  weight  of  the  lower  layers  of  air  near  the  ground. 
The  ammonia  in  the  air  is  about  the  ^nroWolF  o*  ^  ky  weight. 

Bacteria.  —  A  highly  important  constituent  of  the  air,  as  regards  the 
well-being  of  humanity,  is  the  minute  vegetable  organisms  contained, 
floating  around  in  it  as  dust,  known  as  bacteria.  These  are  the  lowest 
forms  of  vegetable  life.  Over  the  oceans  and  on  high  mountains  they 
are  present  in  air  to  the  extent  of  about  one  in  every  cubic  yard  of 
air.  In  the  streets  of  a  city  there  are  about  3000  to  the  cubic  yard ; 


METEOROLOGY. 


in  hospital  wards  there  are  as  many  as  80,000  in  the  same  space.  A 
few  of  the  forms  of  these  are  shown  in  Fig.  i  magnified  500  diam- 
eters. Only  a  small  proportion  of  the  bacteria  are  disease  germs. 
The  remainder  are  workers  that  feed  on  animal  and  vegetable  waste, 
resolving  it  into  simpler  compounds  to  be  absorbed  again  by  the  higher 
members  of  the  vegetable  kingdom. 

Ozone.  —  Ozone,  an  allotropic  form  of  oxygen,  its  density  being  1.5 
times  that  of  ordinary  oxygen,  is  present  in  the  air  in  small  quantities. 
Ozone  is  produced  in  passing  electric  sparks  through 
a  tube  containing  dry  oxygen,  also  by  the  electric 
decomposition  of  water  and  by  the  slow  burning  of 
phosphorus  in  moist  air.     Its  presence  in  the  atmos- 
phere is  supposed  to  be  due  to  lightning.     Ozone 
is  a  more   powerful  oxidizing  agent  than  ordinary 
oxygen.     An  indigo  solution  shaken  up  in  air  con- 
taining  much   ozone   soon   loses   its   colour.      The 
9   /          ,,,t~      usual  test  for  the  presence  of  ozone  is  the  property 
~y.^  ft  nas  °f  decomposing  iodide  of  potassium.     Papers 

soaked  in  a  solution  of  this  become  discoloured  by 
the  iodine  set  free  on  exposure  to  the  air  if  there  is 
much  ozone  present.  The  discoloration  is,  however,  not  a  good  meas- 
ure of  the  amount  of  the  ozone  in  the  air,  being  hard  to  estimate  by 
comparison  with  a  scale  of  colours.  The  amount  of  discoloration 
depends,  moreover,  on  the  strength  of  the  wind  ;  the  varying  velocity 
at  different  times  causes  different  quantities  of  ozone  to  come  in  con- 
tact with  the  paper. 

Nitric  acid,  of  which  there  is  some  trace  in  the  air,  produces  the  same 
sort  of  discoloration  in  iodide  of  potassium  paper  as  ozone. 

There  is  a  decided  discoloration  of  iodide  of  potassium  paper  in 
damp  and  especially  in  rainy  weather,  due,  probably,  to  moisture  facili- 
tating the  access  of  ozone  to  the  iodide  of  potassium,  and  not  to  the 
presence  of  any  greater  amount  than  is  ordinarily  contained. 

Heat  of  Air.  —  The  sun  heats  the  surface  of  the  earth ;  the  air  in  con- 
tact with  it  becomes  heated  and  rises,  on  account  of  its  lightness  as 
compared  with  air  higher  up.  By  mixture  with  other  air  as  it  rises,  the 
heat  received  at  the  surface  of  the  earth  is  diffused  throughout  the 


FIG.  i. 


THE  AIR.  5 

upper  air  by  convection.  About  one-third  of  all  the  heat  received  by 
the  air  is  absorbed  directly  from  the  rays  of  the  sun  in  passing  through 
it.  The  amount  absorbed  varies  with  the  condition  of,  the  air  as  to 
cloudiness,  haziness,  and  humidity. 

Insolation.  — The  direct  action  of  the  sun's  rays  in  heating  the  surface 
of  the  earth  is  called  "insolation."  When  the  sun  is  low  in  the  horizon 
the  heating  is  less  than  when  at  a  height,  on  account  of  the  inclination 
of  the  rays  and  on  account  of  the  greater  thickness  of  air  traversed  be- 
fore reaching  the  earth.  At  an  altitude  of  five  degrees  the  sun's  rays 
come  through  a  thickness  of  air  five  times  as  great  as  when  the  sun  is 
in  the  zenith;  with  the  sun  in  the  horizon  the  thickness  is  35  times  as 
great.  A  surface  exposed  vertically  to  the  sun's  rays  at  the  upper  limit 
of  the  atmosphere  receives  in  one  minute  of  time  about  3.0  gramme- 
calories  of  heat  on  every  square  centimetre  of  surface.  This  quantity 
is  called  the  solar  constant.  A  gramme-calorie  of  heat  is  the  quantity 
required  to  raise  a  cubic  centimetre  of  water  (one  gramme)  one  degree 
centigrade.  A  centimetre  is  0.3937  of  an  inch.  A  degree  centigrade 
is  1.8°  Fahrenheit. 

The  amount  of  heat  received  at  a  place  depends  on  the  duration  of  sun- 
shine and  the  inclination  of  the  sun's  rays  to  the  ground  during  the  time. 

Earth.  — The  earth  is  a  flattened  sphere.  Its  diameter  at  the  equator 
is  7926.6  miles  ;  at  the  poles,  7899.0  miles.  The  time  the  earth  takes  to 
make  one  complete  revolution  is  conventionally  divided  into  24  hours. 
The  velocity  of  a  point  on  the  equator  due  to  the  rotation  of  the  earth 
is  1040  miles  an  hour ;  at  latitude  60°  it  is  one-half  as  much. 

The  earth  moves  around  the  sun  in  an  ellipse  once  in  365.24  days, 
with  the  sun  in  the  focus.  The  plane  of  the  ellipse  is  the  ecliptic. 
The  distance  of  the  earth  from  the  sun  at  its  greatest  is  94,353,000 
miles,  on  June  21,  and  at  its  least,  91,241,000  miles,  on  December  23. 
In  its  course  around  the  sun  the  earth  moves  with  a  velocity  on  the 
average  of  about  65,940  miles  an  hour,  slower  in  June  and  faster  in 
December. 

The  plane  of  the  earth's  equator  forms  an  angle  of  about  23°  27'  with 
the  ecliptic,  known  as  the  obliquity  of  the  ecliptic. 

The  intersections  of  the  ecliptic  with  the  equator,  where  the  sun 
passes  from  one  side  of  the  equator  to  the  other  in  spring  and  autumn, 


6  METEOROLOGY. 

are  called  the  spring  and  autumn  equinoxes.  From  the  spring  to 
the  autumn  equinox  is  184  days,  and  from  the  autumn  to  the  spring 
181. 

Owing  to  the  obliquity  of  the  ecliptic,  the  duration  of  day  and  night  is 
of  varying  length  at  different  times  of  the  year.  At  the  pole  the  sun  is 
above  the  horizon  half  the  year  and  below  half. 

Amount  of  Heat.  —  The  amount  of  heat  received  from  the  sun  is  dif- 
ferent at  different  places,  and  varies  at  different  times  of  the  year  at  the 
same  place.  Taking  the  average  amount  of  heat  received  on  a  surface 
at  the  equator  in  a  day  as  unity,  the  quantity  received  in  a  year  is  365  ; 
at  latitude  30°  it  is  321 ;  at  40°  it  is  288;  at  50°  it  is  250;  at  60°  it  is  208; 
at  the  pole,  152.  The  amount  received  at  the  equator  in  one  day,  March 
20,  being  unity,  the  sun  being  at  the  equinox,  at  30°  it  is  0.88 ;  at  60°, 
0.50  ;  and  at  the  pole,  zero.  The  amount  received  in  a  day  at  the  equator, 
April  12,  is  0.98  ;  at  30°,  0.97  ;  at  60°,  0.69 ;  and  at  the  pole,  0.49.  The 
amount  received,  May  5,  at  the  equator  is  0.99  ;  at  30°,  1.05 ;  at  60°,  0.89; 
and  at  the  pole,  0.86.  The  amount  received,  June  21,  at  the  equator  is 
0.89;  at  30°,  i.io;  at  60°,  i.io;  and  at  the  pole,  1.20. 

The  number  of  thermal  days  at  the  equator  is  365^,  at  the  pole 
151.6. 

Differences  in  the  heating  of  the  air  over  different  parts  of  the  earth 
produce  differences  in  density  which  give  rise  to  motions  of  the  air 
tending  to  restore  equality.  As  there  is  much  more  heat  received  on 
the  average  at  the  equator  than  the  poles,  there  is  a  general  circulation 
of  the  air  from  the  equator  to  the  poles  and  back  again.  As  the  differ- 
ence of  temperature  always  exists,  sometimes  greater  than  at  others,  the 
motion  and  interchange  of  air  between  the  equator  and  the  region  of  the 
poles  is  perpetual.  The  varying  difference  of  temperature  at  different 
times  of  the  year  causes  periodic  oscillations  in  the  intensity  of  this  cir- 
culation. The  difference  of  temperature  is  twice  as  great  in  winter  as 
in  summer,  and  the  upper  currents  twice  to  four  times  as  great.  The 
average  difference  of  temperature  between  the  equator  and  poles  is 
81°  F.  A  surface  of  equal  pressure  at  the  equator  and  poles  is  one- 
sixth  of  its  altitude  higher  at  the  equator  than  the  poles.  This  is  the 
slope  or  gradient  which  in  the  distance  of  6000  miles  gives  the  air  its 
motion. 


THE  AIR.  7 

The  average  weather  or  climate  at  a  place  depends  on  the  varying 
amount  of  heat  received  from  the  sun  at  different  times  of  the  year,  and 
on  the  direction  of  the  prevailing  winds.  The  most  potent  factor  is 
distance  from  the  equator. 

The  main  cause  of  seasonal  variation  of  climate  over  the  earth  is  the 
obliquity  of  the  ecliptic.  The  obliquity  is  at  present  diminishing  at  the 
rate  of  0^.4  a  year.  The  least  value,  22°  15',  will  be  reached  in  about 
15,000  years.  The  greatest  value  in  the  past  has  been  24°  50'. 5.  This 
oscillation  cannot  be  productive  of  more  than  very  slight  secular  varia- 
tions in  seasonal  climate. 

The  differences  in  surface-heating  over  land  and  ocean  have  an 
appreciable  influence  on  the  general  circulation  of  the  air.  Over  the 
ocean  the  sun's  heat  penetrates  the  water  to  a  considerable  depth,  instead 
of  being  concentrated  at  the  surface  as  in  the  case  of  land.  The  specific 
heat  of  the  land  surface  is  only  0.2  that  of  water. 

The  distribution  of  land  and  water  modifies  climates.  Winds  are  more 
potent  than  the  direct  action  of  the  sun  in  making  climate.  In  the 
arctic  region  the  climate  is  more  the  direct  result  of  the  sun's  action 
than  elsewhere. 

Area  of  Earth.  —  The  area  of  the  whole  surface  of  the  earth  is 
196,662,892  square  miles.  About  three-tenths,  or  52,500,000  square 
miles  (26f  per  cent),  is  land.  The  area  between  latitude  25°  north  and 
25°  south  comprises  0.4  of  the  earth's  surface  ;  from  latitude  25°  to 
60°  on  both  sides  of  the  equator  comprises  0.5  ;  the  polar  regions 
above  60°  comprise  o.  i  ;  the  oceanic  islands  form  0.007  °f  *ne  land 
surface  of  the  globe. 

The  eccentricity  of  the  earth's  orbit  is  now  increasing ;  it  was  at  its 
least  value  50,000  years  ago.  When  at  its  greatest,  the  earth  at  aphelion 
is  8,500,000  miles  farther  away  from  the  earth  than  when  the  eccen- 
tricitj-  is  at  its  least.  This,  it  is  believed  by  some,  would  cause  the 
mean  temperature  of  the  northern  hemisphere  to  be  forty-five  degrees 
lower  during  the  winter  half  of  the  year  than  it  is  now. 

A  flow  of  air  takes  place  in  the  upper  atmosphere  from  the  equator 
toward  the  poles,  and  a  counter-current  sets  in  along  the  surface  of  the 
earth  from  the  poles  toward  the  equator.  There  must  be  neutral  surfaces 
where  there  is  no  motion.  In  the  upper  northerly  current  in  the 


8  METEOROLOGY. 

northern  hemisphere  the  greatest  velocity  is  in  the  upper  portion,  and 
it  diminishes  to  nothing  at  the  neutral  surface.  For  the  lower  southerly 
current  the  greatest  velocity  is  at  some  distance  above  the  surface  of 
the  earth,  diminishing  above  to  the  neutral  surface,  and  also  below  to 
the  earth's  surface  on  account  of  friction.  As  the  upper  currents  go 
northward,  the  space  they  occupy  is  less  as  the  meridians  converge.  The 
rotation  of  the  earth  modifies  the  motion,  deflecting  currents  to  the 
right  in  the  northern  hemisphere,  and  to  the  left  in  the  southern  hemi- 
sphere. The  motion  of  the  air  in  middle  and  high  latitudes  is  from  west 
to  east,  and  the  velocity  increases  with  the  altitude.  In  low  latitudes 
the  air  has  a  motion  from  east  to  west  at  the  surface  which  decreases  up 
to  a  certain  altitude,  where  the  motion  changes  to  the  east  and  then 
increases  with  the  altitude.  Deflecting  forces  arising  from  the  easterly 
motion  in  middle  latitudes  push  the  air  towards  the  equator.  A  con- 
trary but  weaker  force,  the  effect  of  the  westerly  motion  in  low  latitudes, 
pushes  the  surface  air  poleward,  the  result  being  to  heap  up  the  air  and 
increase  the  pressure  at  the  surface  at  latitude  thirty  degrees,  while  in 
the  upper  air  there  is  a  maximum  of  pressure  at  the  equator  and  a  mini- 
mum at  the  poles. 

Viewed  as  a  whole,  the  general  circulation  of  the  air  is  two  huge 
whirls,  one  in  each  hemisphere,  with  the  poles  as  centres.  The  direc- 
tion of  motion  is  determined  by  the  rotation  of  the  earth.  In  the 
northern  hemisphere  it  is  opposite  to  the  motion  of  the  hands  of  a  clock, 
and  the  reverse  in  the  southern  hemisphere.  Each  of  the  whirls  has  on 
its  outer  circumference  an  atmospheric  ring,  in  which  the  motion  of 
rotation  of  the  air  is  in  the  opposite  direction. 

In  the  winter  of  the  northern  hemisphere  the  sun  is  nearer  the  earth 
than  in  summer.  The  greater  intensity  of  insolation  of  the  northern 
hemisphere  over  that  of  the  southern  is  counterbalanced  by  its  shorter 
continuance,  so  that  the  same  amount  of  heat  is  received  by  both  in 
the  course  of  a  year. 

Trade-winds.  —  At  the  surface  of  the  earth  both  on  land  and  sea  from 
latitude  f  to  29°  north  the  wind  blows  almost  constantly  from  the  north- 
east all  the  year  round.  From  latitude  3°  to  20°  south  the  wind  is 
almost  constantly  from  the  south-east.  These  currents  of  air  are  known 
as  the  trade-winds.  In  summer  they  prevail  over  a  region  a  few  degrees 


THE  AIR.  9 

farther  north  than  in  winter.  In  the  northern  half  of  the  belt  of 
north-east  trade-winds  the  average  direction  of  the  wind  is  from  a  point 
60°  east  of  north ;  near  latitude  10°  the  direction  is  more  from  the  east, 
and  near  the  southern  limit  almost  exactly  east.  At  the  Hawaiian 
Islands,  latitude  21°  north,  the  trade-wind  prevails  258  days  in  the  year. 
The  days  when  it  does  not  blow  are  mainly  from  December  to  April. 

The  trade-winds  extend  through  a  height  of  12,000  feet  :  the  height 
is  less  near  the  northern  limit  of  the  north-east  trades  than  near  the 
equator. 

Anti-Trade- winds.  —  Above  the  north-east  trade- winds  the  higher 
currents  of  the  air  are  from  the  south-west,  and  are  known  as  the 
anti-trade-winds ;  in  the  southern  hemisphere  they  are  from  the  north- 
west. The  direction  of  the  anti-trade-winds  has  been  shown  by  the 
smoke  and  ashes  of  volcanoes  carried  far  to  the  north-east  several 
hundreds  of  miles  from  the  place  of  origin. 

Calms.  —  Between  the  north-east  and  south-east  trade-winds  there  is 
a  belt  of  calms  varying  in  width  from  150  to  500  miles.  On  the 
Atlantic  Ocean  the  width  is  less  on  the  American  than  the  European 
side,  and  wider  in  the  middle  than  on  either  side.  The  width  is  less 
over  the  Pacific  than  the  Atlantic  Ocean.  The  average  position  of  the 
middle  of  the  belt  of  calms  is  in  latitude  5°  north ;  it  changes  with 
the  declination  of  the  sun,  being  at  10°  north  in  August  and  at  i°  in 
February.  The  belt  of  calms  is  the  region  of  the  ascending  currents 
of  air  where  the  two  systems  of  trade-winds  meet.  The  region  of  calms 
is  known  by  sailors  as  the  "doldrums."  Vessels  have  been  sometimes 
becalmed  in  these  regions  for  as  much  as  six  weeks  at  a  time. 

Horse  Latitudes.  —  At  latitudes  30°  north  and  south  there  are  regions 
of  calms  known  as  the  calms  of  Cancer  and  Capricorn,  marking  the 
region  of  the  descending  currents  in  the  circulatory  system  of  the  trade 
and  anti-trade  winds.  Near  the  West  India  Islands  this  region  was 
once  known  as  the  "Horse  Latitudes." 

Winds  of  Temperate  Zone.  —  Beyond  the  region  of  the  trade-wind 
the  prevalent  direction  of  the  wind  in  the  northern  hemisphere  is  from 
a  point  a  little  south  of  west.  This  zone  of  winds  is  2000  miles  in 
width.  The  westerly  motion  is  most  decided  in  the  middle  of  the  belt, 
and  diminishes  on  either  side.  In  the  United  States,  winds  from  the 


10  METEOROLOGY. 

west  are  two  and  a  half  times  as  frequent  as  those  from  the  east.  In 
the  southern  hemisphere  these  winds  blow  with  great  intensity  between 
latitude  40°  and  50°,  where  they  are  called  the  "brave  west  winds,"  and 
the  latitudes  are  sometimes  known  as  the  "roaring  forties." 

Above  the  south-west  winds  of  the  temperate  zone  the  wind  at  a 
height  is  from  the  north-west.  In  ascending  through  the  air,  the  direc- 
tion gradually  changes  from  the  south-west  around  by  the  west  to  the 
north-west.  Clouds  prevail  principally  in  the  lower  half  of  the  air,  and 
have  generally  the  direction  of  the  lower  air  currents.  When  the  air 
is  very  dry  and  such  cirrus  clouds  as  can  be  seen  are  at  a  great  height, 
they  are  generally  observed  to  come  from  west  8°  north  in  winter,  and 
west  3°  south  in  summer,  in  Canada.  In  Europe  the  direction  is  gen- 
erally from  the  north-west  or  south-west  more  from  the  north  in 
winter. 

Polar  Winds.  —  Above  latitude  60°  in  the  polar  regions  the  general 
tendency  is  for  ground  surface  winds  to  blow  from  the  south-east. 

The  general  tendency  of  upper  currents  is  from  the  west  all  over  the 
world. 

On  account  of  the  different  velocities  of  the  earth  on  different  par- 
allels of  latitude,  air  starting  from  the  equator  and  moving  northward 
has  a  greater  component  of  motion  than  the  place  toward  which  it  is 
moving.  Consequently  the  current  deviates  toward  the  east,  and  the 
easterly  deviation  is  greater  the  farther  north  it  goes.  Currents  in 
the  northern  hemisphere  are  always  deflected  to  the  right  of  one  looking 
in  the  direction  the  current  is  going,  no  matter  what  the  direction. 

Ocean  Currents.  —  Ocean  currents  control  very  largely  the  distribu- 
tion of  surface  temperature  of  the  water.  The  currents  are  mainly  due 
to  the  prevailing  winds  over  the  surface  and  the  difference  of  density 
induced  by  differences  of  temperature  in  the  equatorial  and  polar 
regions.  The  winds  being  the  result  of  the  distribution  of  air  pres- 
sure, the  pressures  in  a  measure  should  represent  the  currents.  There 
is  a  similarity  in  the  average  annual  curves  of  isobars  and  the  general 
circulation  of  the  five  great  oceans. 

The  circulation  is  limited  and  controlled  very  largely  by  the  shape  of 
the  continents.  The  rotation  of  the  earth  has  the  effect  of  deviating 
ocean  currents  the  same  as  in  the  case  of  the  winds,  but  much  less,  as 


THE  AIR.  II 

the  velocities  are  smaller.  The  average  depth  of  the  ocean  is  about 
3000  feet. 

There  are  minor  modifications  of  currents  due  to  difference  in  density 
caused  by  evaporation,  producing  concentration  of  the  salt  in  some  places, 
and  rainfall  diluting  the  surface  salt  water  in  others.  This  is  of  most 
importance,  however,  in  the  vertical  circulation  of  the  ocean.  The 
average  density  of  sea-water  as  compared  with  fresh  water  is  1.024.  In 
the  region  of  the  trade-winds  the  density  is  greater  than  the  average  on 
account  of  the  evaporation,  and  in  the  belt  of  calms  less  than  the  aver- 
age on  account  of  the  rainfall. 

The  general  tendency  of  ocean  currents  at  the  surface  is  from  the 
equator  to  the  poles.  The  return  current  from  the  poles  to  the  equator 
is  principally  at  a  depth.  Little  is  known  of  the  currents  at  great  depths 
in  the  ocean  more  than  that  the  rate  of  motion  must  be  exceedingly 
slow,  moving  perhaps  not  more  than  a  few  feet  in  a  day  in  some  places. 

Strong  and  long-continued  wind  may  raise  to  a  very  considerable 
extent  the  level  of  the  water  along  a  coast  to  which  it  is  blowing. 
During  and  after  a  gale  at  sea  a  strong  current  is  usually  noticeable 
flowing  to  leeward.  The  westerly  wind  blowing  over  the  North  Sea 
sometimes  raises  the  water  6. 5  feet  at  Christiana,  Denmark ;  an  easterly 
wind  will  sometimes  depress  it  3.0  feet  below  the  average.  At  Galveston, 
Texas,  a  strong  wind  at  times  will  raise  the  level  of  the  water  in  the  bay 
6  feet. 

Equatorial  Current.  —  The  equatorial  current  of  the  Atlantic  Ocean 
runs  from  the  Gulf  of  Guinea,  on  the  coast  of  Africa,  west  along  the 
equator.  At  the  easternmost  point  of  South  America,  Cape  St.  Roque, 
it  divides.  One  part  turns  to  the  north-west  and  enters  the  Caribbean 
Sea;  the  other  turns  south-west  and  moves  along  the  shore  of  South 
America.  This  is  called  the  "  Brazil  current." 

Gulf  Stream.  —  The  part  of  the  equatorial  current  that  enters  the 
Caribbean  Sea  passes  through  it  and  between  Yucatan  and  the  island 
of  Cuba  into  the  Gulf  of  Mexico.  From  the  Gulf  of  Mexico  it  passes 
through  the  Straits  of  Florida  into  the  Atlantic  Ocean,  where  it  is 
known  as  the  "  Gulf  Stream."  The  temperature  of  the  Gulf  Stream  in 
January  is  60°.  The  current  through  the  Straits  of  Florida  has  a  veloc- 
ity of  4  miles  an  hour  and  discharges  986,000,000  cubic  feet  of  water 


12  METEOROLOGY. 

per  second.  The  current  moves  north-east,  and  at  Cape  Hatteras  has 
a  velocity  of  4.4  miles  an  hour.  From  a  region  east  of  the  Banks  of 
Newfoundland  it  moves  to  the  middle  of  the  ocean.  On  the  coast  of 
Europe  it  divides.  Part  moves  along  the  coast  of  Spain  and  Portugal 
towards  the  south,  and  along  the  north-west  coast  of  Africa.  Here  its 
velocity  is  increased  by  the  north-east  trade-winds.  The  current  joins 
in  partially  with  the  equatorial  current,  and  part  goes  farther  east, 
along  the  shore  into  the  Gulf  of  Guinea,  where  it  is  called  the  Guinea 
Stream. 

The  other  part  of  the  Gulf  Stream,  where  it  branches  in  the  North 
Atlantic,  moves  north-east  to  the  north  of  the  British  Islands  and  along 
the  coast  of  Norway.  A  part  goes  to  the  west  of  Iceland  and,  uniting 
with  a  cold  current  coming  out  of  the  Greenland  Sea  between  Green- 
land and  Iceland,  moves  down  the  east  coast  of  Iceland.  To  the  north- 
east of  the  Faroe  Islands  it  unites  with  the  principal  current  up  to  the 
Arctic  Ocean,  where  it  divides  into  two  branches.  One  of  these  flows 
along  the  west  coast  of  Spitzbergen,  and  the  other  farther  east  to  Nova 
Zembla. 

The  elevation  of  the  St.  Louis  city,  Mo.,  plane  of  levels  as  deter- 
mined by  precise  levels  above  the  mean  tide  of  the  Gulf  of  Mexico  at 
Biloxi,  Miss.,  is  412.71  feet.  The  elevation  of  the  same  point  deter- 
mined by  levels  from  the  mean  tide  at  Sandy  Hook,  near  New  York 
city,  is  416.36  feet.  The  results  in  both  cases  depend  on  duplicate  sys- 
tems of  precise  levellings.  Part  of  this  difference  of  elevation  is  due  to 
unavoidable  errors  of  observation  in  such  long  lines  of  levels,  and  part, 
perhaps,  to  the  disturbing  influence  on  the  level,  in  crossing  the 
Allegheny  Mountains,  produced  by  slight  variations  in  the  direction 
of  force  of  gravity  from  the  true  centre  of  curvature  of  the  earth.  But 
the  main  part  is  difference  in  water  level  between  the  Gulf  of  Mexico 
and  the  North  Atlantic  Ocean  due  to  the  surface  slope  which  is  the 
cause  of  the  flow  of  the  Gulf  Stream  water. 

In  the  South  Atlantic  Ocean  the  Brazil  current  turns  east  at  lati- 
tude 40°  and  moves  towards  the  coast  of  Africa.  It  continues  to  the 
east  to  the  south  of  the  Cape  of  Good  Hope,  and  nearly  as  far  as  Aus- 
tralia, and  then  turns  and  moves  north-east.  It  is  joined  by  the  equato- 
rial current  from  the  north  that  comes  down  east  of  Madagascar  Island. 


THE  AIR.  13 

Under  the  influence  of  the  south-east  trade-wind  it  joins  in  with  the 
equatorial  current. 

Pacific  Ocean  Currents. —  In  the  Pacific  Ocean  there  are  currents  cor- 
responding to  those  in  the  Atlantic.  The  one  corresponding  to  the  Gulf 
Stream  passes  along  the  coast  of  Japan,  where  it  is  known  as  the  "  Kuro 
Siwa,"  meaning  a  deep  blue  colour.  This  colour  is  also  characteristic  of 
the  water  of  the  Gulf  Stream. 

Kuro  Siwa. — The  Kuro  Siwa  carries  a  great  deal  of  heat  to  the 
shores  of  Alaska  and  British  North  America.  It  turns  south  and  moves 
along  the  coast  of  the  United  States,  bringing  relatively  cold  water 
from  the  north  to  the  southern  coast  of  California. 

Humboldt  Current.  —  From  latitude  40°  south  in  the  Pacific  Ocean  a 
cold  current,  called  the  "Humboldt  current,"  passes  along  the  west 
coast  of  South  America  and  joins  in  with  the  equatorial  current. 

Mozambique  Current.  —  In  the  Indian  Ocean  the  Mozambique  current 
flows  between  the  island  of  Madagascar  and  Africa.  Its  continuation 
along  the  coast  to  the  end  of  the  continent  is  called  the  Algulhas  cur- 
rent. On  the  west  side  of  Africa  it  continues  to  the  north-east  and 
joins  in  with  the  equatorial  current. 


CHAPTER    II. 

METEOROLOGICAL  INSTRUMENTS. 

PROPERTIES  and  conditions  of  the  air  not  discernible  to  the  unaided 
senses  are  observed  with  the  aid  of  instruments.  Appearances  and 
conditions  of  the  air  that  are  discernible  cannot  always  be  estimated 
precisely  enough  for  purposes  of  comparison  with  like  conditions  at 
other  times  and  places,  and  instruments  are  devised  for  measuring  them 
accurately. 

The  most  important  conditions  of  air  are  :  — 

Temperature,  measured  with  thermometers. 

Sun's  rate  of  heating,  measured  with  actinometer. 

Pressure,  measured  with  barometer. 

Rainfall,  measured  with  rain-gauge. 

Soil-water  reaching  different  depths  in  the  earth,  measured  with  a  percola- 
tion gauge. 

Snowfall,  melted,  measured  with  a  rain-gauge. 

Snowfall,  unmelted,  measured  with  a  rod. 

Vapour  pressure,  measured  with  psychrometer,  or  dew-point  apparatus. 

Wind  direction,  observed  by  a  wind-vane. 

Wind  velocity,  measured  by  anemometer. 

Cloud  motion,  observed  with  a  nephoscope. 

Cloudiness,  amount  in  fractional  part  of  the  sky  covered,  estimated  with 
the  eye. 

Sunshine  duration,  measured  with  a  sunshine  recorder. 

Electric  potential,  measured  with  an  electrometer. 

Evaporation,  measured  with  an  evaporometer. 

Fog,  haze,  estimated. 

In  the  observation  of  rivers  the  important  conditions  are  :  — 

River  stages,  height  of  river-surface  in  feet  above  low  water,  measured  by 

a  river-gauge. 

Current  velocity,  measured  with  a  current-metre,  or  ship's  log. 

14 


METEOROLOGICAL  INSTRUMENTS. 


THERMOMETER. 

The  sensation  of  heat  and  cold  in  air  and  objects  is  called  tempera- 
ture, and  is  measurable  with  a  thermometer.  The  most  convenient 
and  best  kind  is  the  well-known  mercurial  thermometer,  consisting  of  a 
glass  bulb  and  stem  containing  mercury  sealed  off  from  the  air. 

Mercury  expands  with  changes  of  temperature  seven  times  as  much 
as  glass.  The  height  of  the  top  of  column  of  mercury  in  the  stem  of 
thermometer  varies  with  the  temperature. 

Scale.  —  On  the  Fahrenheit  scale  the  temperature  of  the  melting- 
point  of  ice  is  taken  as  32°,  usually  called  freezing-point,  and  the  boiling- 
point  of  water  as  212°.  The  temperature  of  melting  ice  varies  slightly 
with  the  pressure  of  the  air.  One  atmosphere  of  pressure  lowers  the 
temperature  by  0.0135  of  a  degree.  But  an  increase  of  that  amount  of 
pressure  on  the  bulb  of  a  thermometer  will  compress  it  to  such  an  ex- 
tent that  the  column  of  mercury  will  stand  0.5  of  a  degree  higher  than 
without  the  pressure. 

The  tube  of  a  thermometer  is  closed.  When  the  top  is  broken, 
admitting  the  pressure  of  the  air,  it  lowers  the  reading  of  the  ther- 
mometer 0.5  of  a  degree. 

Boiling-point.  —  The  temperature  of  boiling-point  varies  with  the 
atmospheric  pressure.  Doubling  the  pressure  raises  the  boiling-point 
to  250°.  A  change  in  the  pressure  of  one-tenth  of  an  inch  changes  the 
temperature  of  boiling-point  0.17  of  a  degree.  The  pressure  for  which 
the  temperature  of  boiling-point  is  212°  is  29.922  inches  of  mercury  at 
the  temperature  of  freezing-point,  subject  to  the  force  of  gravity  at  the 
level  of  the  sea  and  latitude  45°. 

Calibration. — The  contents  of  a  thermometer  tube  from  32°  to  212° 
is  divided  into  180  equal  parts  by  the  graduations.  The  tube  of  a  ther- 
mometer is  of  variable  diameter  from  point  to  point.  If  the  bore  was 
uniform,  a  division  of  the  scale  to  equal  parts  would  correspond  to  equal 
volumes.  This  never  being  the  case,  the  positions  of  the  marks  indi- 
cating equal  volumes  have  to  be  ascertained  by  a  calibration.  Even 
when  a  thermometer  is  calibrated  by  a  maker,  which  is  usually  the  case, 
the  small  outstanding  errors,  where  great  refinement  is  required,  as  in 
the  case  of  standard  thermometers,  have  to  be  determined  by  a  process 


16  METEOROLOGY. 

of  calibration.  Calibration  is  effected  by  means  of  the  measured 
lengths  of  detached  threads  of  mercury  in  different  parts  of  the  bore 
of  the  stem,  in  terms  of  the  scale  on  the  stem. 

Poggendorf  Correction.  —  The  calibration  of  a  thermometer  being 
made  for  the  stem  at  a  uniform  temperature,  the  graduation  will  not 
correspond  exactly  to  equal  volumes  when  used  at  different  tempera- 
tures. This  correction  for  capacity  of  stem  to  reduce  to  a  uniform 
temperature  is  very  small.  At  its  greatest,  in  the  middle  of  the  scale 
at  a  temperature  of  122°,  it  causes  a  thermometer  to  read  0.12  of  a 
degree  too  high.  This  is  called  the  "  Poggendorf  correction,"  from  the 
name  of  the  physicist  who  suggested  it. 

Reduction  to  Gas-Thermometer.  —  Glass  and  mercury  expand  slightly 
more  at  a  high  than  a  low  temperature  for  the  same  increase  of  tem- 
perature. On  this  account  there  is  a  correction  to  a  thermometer,  to 
reduce  its  indications  to  what  they  would  be  if  the  expansion  of  the 
materials  was  perfectly  uniform.  The  main  part  of  the  correction  is 
due  to  the  variable  expansion  of  the  glass  and  not  the  mercury. 
Thermometers,  from  this  cause,  most  generally  read  too  high.  The 
amount  of  the  correction  varies  for  thermometers  of  different  kinds  of 
glass.  It  is  least  for  hard  glass  containing  but  little  oxide  of  lead  or 
manganese.  At  about  100°,  where  the  correction  is  greatest,  it  amounts 
to  0.19  of  a  degree,  and  diminishes  above  and  below  that  point. 

This  correction  is  derived  by  comparison  of  the  mercurial  thermom- 
eter with  a  gas-thermometer.  A  glass  or  platinum  bulb  is  filled  with 
gas  kept  at  a  constant  volume  at  the  different  temperatures,  except  in 
as  far  as  the  volume  of  the  bulb  changes  with  temperature.  The  pres- 
sure of  the  gas  at  freezing  and  boiling  point  is  observed  by  means  of 
an  attached  manometer.  At  any  other  pressure  the  temperature  is  a 
proportional  part  of  the  interval  from  freezing-point  to  boiling-point 
corresponding  to  the  pressure. 

The  gas-thermometer  adopted  by  the  International  Bureau  of  Weights 
and  Measures  is  hydrogen  in  a  platinum  bulb  with  the  gas  at  a  pressure 
of  39.37  inches  of  mercury  (one  metre)  at  freezing-point. 

The  hydrogen  gas-thermometer  reads  at  104°,  0.018  of  a  degree  higher 
than  one  filled  with  nitrogen,  and  o.  1 08  of  a  degree  higher  than  one  filled 
with  carbon  dioxide. 


METEOROLOGICAL  INSTRUMENTS.  \*J 

The  indications  of  a  gas-thermometer  require  correction  to  reduce  to 
a  perfect  gas,  that  is,  a  gas  in  which  the  increase  of  pressure  is  strictly 
proportional  to  the  increase  of  temperature.  This  correction,  which,  at 
its  greatest  in  the  middle  of  the  scale,  cannot  amount  to  more  than  0.03 
or  0.04  of  a  degree,  is  called  the  reduction  to  the  therm o-dynamic  stand- 
ard. No  satisfactory  method  of  making  this  correction  has  as  yet  been 
devised,  and  it  is  customarily  omitted  even  in  the  most  accurate  investi- 
gations. 

Freezing-point.  —  There  is  a  constant  rise  in  the  freezing-point  of  a 
thermometer  with  age,  due  to  a  contraction  of  the  glass  bulb.  A  ther- 
mometer graduated  the  day  it  is  filled  will  read  a  degree  and  a  half 
higher  at  freezing-point  a  week  after,  when  packed  in  melting  ice.  Fine 
instruments  are  not  graduated  until  a  year  and  a  half  after  filling. 
Even  then  the  rise  in  the  freezing-point  in  the  course  of  six  years  will 
often  be  as  great  as  half  a  degree. 

To  obtain  true  temperatures,  any  change  in  the  freezing-point  after 
an  instrument  is  standardized  must  be  applied  as  a  constant  correction 
for  every  temperature  along  the  scale  being  in  the  nature  of  an  index- 
correction.  Subjecting  a  thermometer  to  the  temperature  of  boiling- 
point  depresses  the  freezing-point  by  an  amount  varying  from  a  third  to 
a  half  degree,  depending  on  the  kind  of  glass  of  which  the  bulb  is  made. 
It  regains  its  former  freezing-point  reading  in  the  course  of  about  a 
month. 

Subjecting  a  thermometer  to  a  very  high  temperature,  as  500°,  raises 
the  freezing-point  permanently  from  10  to  18  degrees. 

Subjecting  a  thermometer  to  a  very  low  temperature,  as  —40°,  raises  the 
freezing-point  a  few  hundredths  of  a  degree.  Exposure  to  low  tempera- 
tures must  be  continued  for  several  hours  to  make  any  appreciable  effect. 
A  coating  of  ice  on  the  bulb  at  low  temperature,  by  compressing  it,  may 
make  the  column  read  a  few  tenths  too  high. 

A  coating  of  copper,  silver,  or  gold  electrically  deposited  on  a  bulb, 
compresses  it  so  as  to  make  the  column  read  several  degrees  too  high, 
when  the  coating  is  thick.  This  effect  has  been  called  electro-striction. 

The  position  of  freezing-point  of  a  thermometer  to  be  used  in  correct- 
ing any  observed  temperature  is  the  freezing-point  observed  immediately 
after  exposure  to  the  temperature. 


i8 


METEOROLOGY. 


In  observing  the  freezing-point  of  a  thermometer  with  melting  snow 
taken  from  air  at  a  temperature  below  freezing-point,  it  gives  the  freez- 
ing-point too  low  unless  saturated  with  water. 

Standard  Thermometers.  —  A  standard  thermometer,  carefully  investi- 
gated and  all  its  corrections  applied  as  described  above,  can  be  made  to 
give  temperatures  with  an  accuracy  of  two  one-hundredths  of  a  degree 
from  o°  to  100°.  From  o°  to  —  39°.8,  the  melting-point  of  mercury,  the 
mercurial  thermometer  is  trustworthy  to  0.05  of  a  degree. 

Common  Thermometers.  —  Ordinary  thermometers  are  graduated  by 
reference  to  the  temperature  of  freezing-point  in  melting  ice,  and  by 
comparison  with  a  standard  at  the  temperatures  of  52°,  72°,  and  92°,  the 

intermediate  spaces  being  divided  into 
equal  parts.  Above  the  temperature 
of  32°  comparisons  with  a  standard 
are  made  with  the  thermometers  in 
water  well  mixed  to  keep  it  uniform 
in  temperature  throughout.  Below 
32°,  and  down  to  —28°,  comparisons 
are  made  in  alcohol  cooled  down  by 
means  of  liquefied  ammonia  escaping 
from  a  reservoir  through  a  coil  in  a 
vessel  surrounding  it  and  containing 
the  alcohol.  From  -28°  to  -68°  the 
cooling  of  alcohol  for  comparing  ther- 
mometers is  accomplished  by  means 
of  evaporating  nitrous  oxide,  or  by 
ammonia  in  case  there  is  means  for 
pumping  out  the  ammonia  from  the 
vessel  and  thereby  inducing  a  brisk 
evaporation  sufficient  to  produce  the 
low  temperature.  The  evaporation 
of  ammonia  or  nitrous  oxide  induces 

intense  refrigeration  down  to  - 100°  or  lower.     The  apparatus  for  this 
purpose  is  shown  in  Fig.  2. 

Low  Temperatures.  —  For  temperatures  below  the  freezing-point  of 
mercury  alcohol  thermometers  are  used,  standardized  by  comparison  at 
low  temperatures  with  a  gas-thermometer. 


m 


FIG.  2. 


METEOROLOGICAL  INSTRUMENTS.  19 

Cheap  tin-case  thermometers  that  sell  for  twenty-five  cents  apiece  are 
fitted  with  ready-made  scales.  The  thermometers  are  made  in  large 
numbers.  A  number  of  scales  of  different  lengths  are  made,  and  one 
selected  the  nearest  in  length  to  a  measured  interval  of  temperature  on 
a  tube  to  be  fitted  ;  the  distance  taken  is  usually  from  32°  to  92°.  Such 
thermometers  are  often  in  error  as  much  as  two  degrees  too  high  or 
too  low  at  70°,  and  often  as  much  as  five  degrees  at  —30°,  usually 
giving  temperatures  too  low. 

Maximum  Thermometers.  —  The  highest  temperature  occurring  dur- 
ing the  day  is  registered  by  a  maximum  thermometer.  One  form 
consists  of  an  ordinary  thermometer,  with  the  bore  of  the  tube  near  the 
bulb  contracted,  so  that  when  cooling  begins  the  thread  of  mercury 
separates,  leaving  the  top  of  the  column  at  the  highest  temperature 
reached.  It  is  reset  for  observation  again  by  whirling  it  on  a 
pivot  at  the  top  of  the  metal  strip  to  which  the  tube  is  attached. 
The  centrifugal  force  developed  drives  the  mercury  back  into  the 
bulb. 

When  a  thermometer  of  this  sort  is  not  read  for  several  hours  after 
the  highest  temperature  occurs,  by  which  time  the  air  may  have  cooled 
down  fifteen  or  twenty  degrees,  the  highest  recorded  temperature  may 
be  half  a  degree  too  low  from  contraction  of  the  detached  thread  of 
mercury  in  cooling. 

When  the  fall  of  temperature  is  very  slow,  a  little  of  the  mercury 
will  pass  down  at  times  before  the  thread  breaks,  especially  when  there 
is  no  wind  to  cause  a  slight  jarring  of  the  instrument. 

In  some  instruments  of  this  kind  the  narrowing  of  the  bore  develops 
a  strong  capillary  action,  and  when  the  connection  of  the  column  is 
broken,  the  detached  thread  moves  up  a  little,  and  causes  the  ther- 
mometer to  register  somewhat  too  high.  This  is  apt  to  occur  in 
winter,  when  the  maximum  temperatures  are  low  and  the  detached 
thread  short.  It  also  occurs  with  long  threads  when  the  thermometer 
is  nearly  horizontal. 

Anpther  form  of  maximum  thermometer  consists  of  an  ordinary 
mercurial  thermometer  with  a  detached  thread  of  mercury  an  inch  or 
so  in  length  and  an  air-bubble  interposed  between  it  and  the  main 
column  of  mercury.  A  thermometer  of  this  kind  must  have  a  very 


20  METEOROLOGY. 

fine  bore  to  work  well,  otherwise  the  index  or  detached  thread  is  liable 
to  be  lost  by  joining  on  with  the  main  column  of  mercury. 

Minimum  Thermometers.  —  For  obtaining  the  lowest  temperatures 
of  the  day,  a  minimum  alcohol  thermometer  is  used.  An  index,  half 
an  inch  long,  made  of  enamel  is  fitted  loosely  in  the  bore  of  an  alcohol 
thermometer  and  immersed  in  the  liquid.  When  the  temperature  falls, 
the  index  is  carried  along  the  bore  and  the  top  stops  at  the  lowest 
point  reached  by  the  top  of  the  alcohol  column.  It  is  reset  for  an- 
other observation  by  raising  the  bulb  end  of  the  thermometer  and 
allowing  the  index  to  slide  down  the  bore  to  the  end  of  the  column, 
where  it  stops.  Jarring  by  the  wind  is  apt  to  displace  the  index,  and 
make  the  instrument  read  too  low. 

Gas-bubbles  are  apt  to  develop  in  the  bulb  or  bore,  and  breaking  the 
continuity  of  the  column,  make  the  instrument  read  too  high.  Small 
portions  of  the  alcohol  become  detached  at  the  top  of  the  column,  and 
remaining  unnoticed  for  a  time,  cause  the  observed  temperatures  to  be 
too  low.  The  formation  of  these  detached  portions  of  alcohol  can 
be  prevented  to  some  extent  by  wrapping  tin-foil  around  the  thermom- 
eter tube. 

Detached  alcohol  can  be  reunited  by  repeatedly  tapping  the  side  of 
the  instrument  lightly  on  a  block  of  wood  with  the  bulb  end  held  down, 
when  the  alcohol  will  gradually  trickle  down  the  tube  in  small  streams. 

An  alcohol  thermometer,  under  the  most  favourable  circumstances, 
will  not  give  temperatures  with  any  greater  accuracy  than  0.6  of  a 
degree.  A  mercurial  thermometer,  when  it  can  be  used,  is  always  pref- 
erable to  one  with  alcohol.  For  temperatures  below  the  freezing-point 
of  mercury,  an  alcohol  thermometer  must  be  used,  and  is  preferable  for 
that  purpose  to  one  filled  with  ether  or  bisulphide  of  carbon,  which 
are  sometimes  used. 

Thermometer  a  Marteau.  —  There  is  a  form  of  alcohol  minimum 
thermometer  in  which  the  index  fits  rather  tightly  in  the  bore  of  the 
tube.  The  tube  contains  a  long  piece  of  enamel  below  the  index 
which  moves  freely  when  the  instrument  is  inclined.  The  index  is  set 
for  a  new  observation  by  hammering  on  the  end  with  the  long  piece  of 
enamel,  whence  the  name  from  the  French  word  "marteau"  for  a 
hammer. 


METEOROLOGICAL  INSTRUMENTS.  21 

Alcohol  thermometers  are  liable  to  read  too  low  by  age,  especially 
if  much  exposed  to  the  direct  light  of  the  sun.  Some  change  takes 
place  in  the  nature  of  the  alcohol  or  the  impurities  contained,  which 
'cause  the  formation  of  a  thin  skin  on  the  inside  of  the  tube  which  can 
be  seen  with  a  microscope.  This  sometimes  causes  a  thermometer  to 
read  a  degree  too  low.  This  masks  the  change  due  to  contraction  of 
the  glass  bulb  with  age.  The  change  of  freezing-point  due  to  change 
in  the  glass  is  usually  inappreciable  in  alcohol  thermometers  on  account 
of  the  comparatively  great  expansion  of  alcohol,  which  is  about  six 
times  that  of  mercury. 

Solar  Radiation  Thermometer.  —  A  radiation  thermometer  shown  in 
Fig.  3  is  a  mercurial  thermometer  enclosed  in  a  glass  tube  from  which 
the  air  is  exhausted.  They  are  commonly  used  in  pairs,  one  with  the 


FIG.  3. 

bulb  blackened  and  the  other  bright.  The  instruments  are  known  as 
bright  and  black  bulb  in  vacuo.  A  black  bulb  will  read  in  the  sun- 
shine from  thirty  to  sixty  degrees  higher  than  the  temperature  of  the 
air  in  the  shade.  The  temperature  to  which  the  black  bulb  rises 
depends  on  the  rate  at  which  heat  is  received  from  the  sun  and  radiated 
to  the  glass  enclosure  around  the  bulb.  The  glass  bulb  enclosure, 
about  one  inch  and  a  half  in  diameter,  is  nearly  at  the  temperature 
of  the  air  in  contact  with  it. 

The   bright   and   black  bulbs  are  intended  to  get  the  difference  in 
heating  effect  of  the  sun  under  different  conditions  of  the  air.     When 


22  METEOROLOGY. 

there  is  much  moisture  or  dust  in  the  air,  more  of  the  sun's  heat  is 
absorbed  than  when  clear. 

Not  much  of  importance  has  yet  been  learned  of  the  physics  of  the 
air  from  the  use  of  these  instruments.  Different  instruments  are  not 
strictly  comparable.  The  results  at  one  place  cannot  be  compared 
with  those  at  another.  The  readings  are  affected  by  the  size  of  the 
thermometer  bulbs,  the  thickness  of  the  film  of  blacking,  the  thickness 
of  the  glass  bulb  of  the  enclosure,  and  on  variations  in  its  nature,  not 
visible  to  the  eye,  some  varieties  of  glass  transmitting  very  much 
more  of  the  sun's  heat  than  others.  Much  also  depends  on  the  per- 
fection of  the  vacuum.  Instruments  the  same  to  all  appearances  may 
indicate  temperatures  different  by  six  degrees  when  exposed  in  the  sun 
side  by  side. 

High  up  in  the  air,  as  on  mountain-tops,  the  radiation  thermometer 
in  the  sunshine  may  read  as  high  as  the  boiling-point  of  water. 

In  the  arctic  regions  in  summer  the  difference  in  bright  and  black 
bulb  is  sometimes  as  great  as  ninety-six  degrees. 

The  name  "radiation  thermometer"  is  also  applied  to  an  alcohol  ther- 
mometer exposed  on  the  ground  in  the  night-time  to  get  the  radiation 
temperature  of  the  soil.  It  is  the  same  as  the  ordinary  minimum 
alcohol  thermometer,  except  that  it  has  a  glass  tube  over  the  scale  to 
keep  the  dew  from  washing  out  the  graduation  marks.  During  the 
night  it  may  take  a  temperature  ten  degrees  lower  than  that  of  the 
air  a  few  feet  above  the  ground  when  the  air  is  still. 

Camphor  Thermoscope.  —  A  sealed  glass  tube  containing  camphor 
gum  dissolved  in  alcohol  is  sometimes  used  to  indicate  changes  of  tem- 
perature. When  the  temperature  is  high,  the  camphor  is  all  dissolved. 
When  the  temperature  falls,  the  camphor  crystallizes  out,  forming  a 
flocculent,  feathery-looking,  whitish  mass.  Thermoscopes  of  this  kind 
are  sometimes  sold  mounted  on  the  same  board  with  a  mercurial  ther- 
mometer, and  are  erroneously  supposed  by  some  to  indicate  changing 
pressure,  or  electrical  condition  of  the  atmosphere. 

Deep  Sea  Thermometers.  —  For  deep  sea  temperatures,  thermometers 
are  used  which  register  on  being  inverted  by  detaching  a  column  of 
mercury.  The  thermometer  is  enclosed  in  a  hermetically  sealed  thick 
glass  tube,  to  protect  it  from  the  pressures  at  great  depths,  which  would 


METEOROLOGICAL  INSTRUMENTS.  2$ 

otherwise  make  it  read  too  high  or  even  crush  the  bulb  to  pieces.  At 
a  depth  of  three  miles  the  pressure  is  two  and  a  half  tons  to  the  square 
inch.  The  best  form  of  the  instrument  is  the  Miller  Casella. 
-  Shape  of  Bulb. — Thermometers  are  made  with  bulbs  of  different 
shapes.  The  most  common  shapes  are  the  cylindrical  and  spherical, 
the  cylindrical  bulb  being  the  most  popular  form  at  the  present  time. 
A  cylindrical  bulb  is  usually  more  sensitive  than  a  spherical  bulb  of  the 
same  volume,  as  it  exposes  more  surface  to  the  air.  Spherical  bulbs 
can,  however,  be  blown  thinner  than  cylindrical  bulbs,  and  the  glass 
is  commonly  more  uniform  in  thickness  than  in  cylindrical  bulbs. 

When  the  temperature  of  the  air  is  changing  rapidly,  as  in  the  case 
of  observation  during  a  balloon  ascent,  a  thermometer  must  be  sensi- 
tive to  give  a  correct  temperature.  In  fact,  a  correction  must  usually 
be  applied  for  sluggishness  in  following  the  temperature  of  the  air 
where  the  greatest  accuracy  is  required. 

Sensitive  Thermometers.  —  A  sensitive  thermometer  has  been  devised 
with  a  bulb  about  the  dimensions  of  a  coarse  needle,  but  shorter.  An 
instrument  of  this  kind  will  give  the  temperature  in  a  minute  as  accu- 
rately as  one  of  the  ordinary  form  in  three  minutes.  A  thermometer 
of  this  sort,  having  a  very  small  quantity  of  mercury,  the  column,  on 
account  of  its  fineness,  is  very  difficult  to  observe. 

Magnifying  Front. — This  is  obviated  by  having  the  bore  of  the 
thermometer  well  to  the  front  of  the  stem,  and  the  front  very  much 
curved.  The  effect  of  this  is,  that  the  column  of  mercury  is  magnified 
in  width  ten  or  twelve  times,  and  can  be  seen  very  distinctly.  Even 
with  the  bore  in  the  centre  of  the  tube,  as  it  is  ordinarily,  the  column 
of  mercury,  as  viewed  through  the  tube,  is  magnified  two  and  a  half 
times. 

Thermometers  are  sometimes  made  with  a  flattened  bore  of  elliptical 
section  for  ease  in  reading.  It  is  not  a  good  form.  When  the  tempera- 
ture is  falling,  the  top  of  the  mercurial  meniscus  is  not  flat  but  slightly 
slanting,  and  introduces  errors  in  reading. 

On  rare  occasions  a  thermometer  will  be  found  of  which  the  indica- 
tions are  very  puzzling  without  any  apparent  cause.  This  is  due  to  the 
thermometer  having  two  bores  in  the  tube  instead  of  one.  The  extra 
bore  is  unintentional,  being  an  accident  of  manufacture,  not  discovered 


24  METEOROLOGY. 

by  the  maker.     The  extra  bore  is  often  back  of  the  enamel  or  in  some 
other  place  equally  difficult  of  discovery. 

Centigrade  Scale. — A  thermometer  with  the  interval  from  freezing- 
point  to  boiling-point  graduated  to  one  hundred  parts,  the  freezing- 
point  corresponding  to  zero,  is  on  the  centigrade  scale,  or  Celsius  scale, 
as  it  is  sometimes  called.  The  centigrade  thermometer  is  in  general 
use  on  the  continent  of  Europe.  The  Fahrenheit  scale  is  in  common 
use  in  all  English-speaking  countries,  the  centigrade  being  only  in  use 
in  laboratory  work  and  in  mathematical  analysis. 

Reaumur  Scale.  —  A  thermometer  graduated  to  the  Reaumur  scale 
has  the  interval  from  freezing-point  to  boiling-point  divided  into  eighty 
parts,  and  the  freezing-point  corresponding  to  zero.  This  scale  is  not 
much  used  any  place  now.  There  are,  however,  many  valuable  old 
temperature  records  in  this  scale. 

De  Lisle  Scale.  —  In  Russia,  a  century  or  more  ago,  the  De  Lisle 
scale  was  in  use  to  some  extent.  The  interval  from  freezing-point  to 
boiling-point  was  divided  into  one  hundred  and  fifty  parts,  and  the 
freezing-point  was  taken  as  zero. 

Thermometer  Shelter.  —  To  get  the  temperature  of  the  air,  the  ther- 
mometer is  set  up  inside  of  a  cubical  wooden  lattice-work  enclosure, 
called  a  "  shelter."  Sometimes  the  cube  is  eighteen  inches  on  a  side. 
The  bottom  is  of  close  boards.  The  roof  is  also  close  and  made  double, 
one  part  being  six  inches  above  the  other  and  open  on  two  sides  to 
admit  of  free  circulation  of  the  air.  The  object  of  a  double  roof  is 
to  prevent  radiation  to  the  thermometer  bulbs  from  the  roof  boards 
heated  directly  by  the  sun's  rays.  The  bottom  of  shelter  is  closed,  to 
prevent  any  effect  on  the  temperature  of  the  thermometer  by  radiation, 
or  upward  warm  currents  from  the  roof  of  a  building  or  the  ground 
where  the  shelter  is  set  up. 

To  get  an  accurate  air  temperature,  a  thermometer  must  be  protected 
from  the  direct  rays  of  the  sun.  There  must  be  free  ventilation  inside 
the  shelter,  or  the  stagnant  air  will  become  heated  from  contact  with 
the  sides,  which  are  heated  by  the  direct  rays  of  the  sun. 

A  shelter  is  usually  placed  ten  feet  above  a  roof  or  the  ground. 

A  shelter  also  protects  a  thermometer  from  radiating  to  the  sky,  and, 
in  a  measure,  from  the  deteriorating  effects  of  rain  and  snow  on  the 


METEOROLOGICAL  INSTRUMENTS. 


instruments.  Radiation  to  the  sky  is  apt  to  make  a  reading  of  temper- 
ature too  low,  especially  in  the  night-time.  The  ground  is  cooler  than 
the  air  in  the  night,  but  the  effect  of  direct  ground  radiation  on  a 
thermometer  is  usually  inappreciable  at  a  greater  height  than  four  feet 
above  the  ground. 

Effects  of  sunshine  and  radiation  on  the  temperatures  shown  by  a 
thermometer  are  almost  entirely  removed  by  whirling  it  for  three  min- 
utes before  reading.  A  thermometer  whirled  in  the  sunshine  gives  a 
temperature  about  half  a  degree  higher  than  in  the  shade. 

A  thermometer  in  quiet  air  in  the  sunshine  may  read  ten  degrees  or 
more  higher  than  the  air  temperature,  depending  on  the  strength  of 
the  wind  prevailing.  If  the  air  is  perfectly  calm,  the  thermometer  may 
even  read  twenty  degrees  higher  than  the  air.  If  the  wind  is  blowing 
twenty  miles  an  hour,  it  will  only  read  a  few  tenths  of  a  degree  higher 
than  in  the  shade.  Moving  particles  of  air,  coming  in  contact  with  the 
bulb,  rapidly  carry  away  by  convection  the  heat  received  from  the  sun. 
When  there  is  no  wind  the  bulb  creates  a  local  atmosphere  of  warm  air 
around  it.  The  thermometer  in  vacua  shows  that  it  is  possible  for 
a  thermometer  to  become  heated 
very  greatly  above  the  temperature 
of  the  air  in  the  sunshine  when  all 
contact  with  the  air  is  shut  off.  The 
record  of  a  thermometer  in  sunshine 
has  therefore  no  climatic  or  meteor- 
ological significance,  for  its  reading 
depends  at  the  same  time  on  the 
strength  of  the  wind  as  well  as  the 
temperature  of  the  air. 

Radiometer.  —  Radiation  of  heat  is 
illustrated  by  the  radiometer  shown  in 
Fig.  4.  Four  light  vanes  of  mica  or 
aluminium  on  two  cross-arms  are  set 
up,  free  to  rotate  in  a  glass  bulb  from  which  the  air  is  almost  entirely 
exhausted.  One  side  of  each  vane  is  blackened.  Black  surfaces 
absorb  heat  better  than  bright  ones.  The  greater  amounts  of  heat 
received  by  the  blackened  sides  as  compared  with  the  bright  ones 


FIG.  4. 


V 


26  METEOROLOGY. 

produce  a  greater  repulsion  on  the  black  sides  between  the  vane  and 
the  tenuous  air  than  on  the  bright  sides.  The  consequence  is  that, 
when  exposed  to  any  source  of  radiant  heat,  as  the  sun  or  a  candle,  there 
is  a  rapid  rotation  of  the  vanes.  A  candle  held  at  a  distance  of  four 
inches  will  produce  a  hundred  and  seventy  revolutions  in  the  time  that 
two  are  produced  with  the  candle  at  a  distance  of  forty  inches. 

When  the  bulb  contains  air  at  the  ordinary  pressure  there  is  no  rota- 
tion of  the  vanes,  as  the  feeble  force  developed  is  not  sufficient  to  over- 
come the  resistance  of  the  dense  air  to  the  motion. 

If  the  air  is  perfectly  exhausted  from  the  bulb,  or  if  it  is  at  least  as 
free  of  air  as  an  incandescent  electric  lamp,  there  is  no  motion  of  the 
vanes.  The  motion  is  at  its  greatest  when  the  pressure  of  the  con- 
tained air  is  about  0.012  of  an  inch. 

The  seat  of  the  reaction  of  the  rotating  vanes  is  not  at  the  source  of 
heat,  —  the  sun  or  candle,  — but  at  the  glass  walls  of  the  bulb,  as  shown 
by  the  fact  that  when  the  bulb  is  floated  on  water  or  delicately  sus- 
pended in  a  vacuum  it  rotates  in  a  direction  opposite  to  that  of  the 
vanes,  but  much  more  slowly  on  account  of  its  greater  mass. 

Actinometer.  — An  actinometer  is  an  instrument  for  determining  the 
amount  of  heat  received  from  the  sun  on  a  surface  of  definite  size  in  a 
given  time.  It  consists  of  a  mercurial  thermometer  with  its  spherical 
bulb  in  the  centre  of  two  concentric  brass  spheres.  The  space  between 
the  spheres  is  filled  with  water  or  chipped  ice,  so  as  to  keep  the  enclos- 
ure around  the  bulb  at  a  uniform  temperature.  An  opening  in  the 
sphere  permits  of  exposing  the  thermometer  bulb  to  the  rays  of  the  sun 
and  cutting  them  off  at  will.  The  thermometer  takes  on  the  tempera- 
ture of  the  enclosure  after  it  is  within  it  for  some  time.  When  the  sun 
shines  on  the  bulb,  its  temperature  rises.  It  warms  up  rapidly  at  first, 
then  more  slowly,  and  finally  the  temperature  becomes  stationary  when 
the  heat  received  from  the  sun  is  exactly  equal  to  the  heat  radiated  by 
the  thermometer  to  the  enclosure.  The  radiation  is  proportional  to  the 
difference  in  temperature  between  the  bulb  and  the  enclosure.  The 
radiation  increases  as  the  temperature  rises,  until  it  becomes  so  great 
as  to  equal  the  rate  at  which  heat  is  received  from  the  sun.  When  the 
sun's  rays  are  cut  off,  the  temperature  of  the  thermometer  falls,  rapidly 
at  first,  and  then  more  slowly  as  its  temperature  approaches  that  of  the 


METEOROLOGICAL  INSTRUMENTS.  2/ 

enclosure.  When  the  sun  is  shining  on  the  thermometer,  its  tempera- 
ture is  increasing  at  a  rate  equal  to  the  difference  between  the  rate  at 
which  heat  is  being  received  by  the  bulb  from  the  sun  and  the  rate 
-at  which  it  is  losing  heat  at  the  same  time  by  radiation  to  the  colder 
enclosure. 

From  a  series  of  readings  of  the  thermometer  at  intervals  of  a  min- 
ute while  exposed  to  the  sun,  and  a  similar  series  while  cooling,  the  law 
of  the  heating  and  cooling  as  dependent  on  the  time  can  be  derived. 

The  rate  of  heating  per  minute,  plus  the  rate  of  cooling  per  minute, 
is  the  rate  at  which  heat  is  being  received  from  the  sun.  This  rate  is 
usually  expressed  in  centigrade  degrees. 

The  dimensions  of  the  bulb  can  be  measured  and  the  quantity  of 
mercury  contained  in  it  computed  and  the  area  of  the  cross-section  of 
the  bulb  ascertained. 

The  unit  of  quantity  of  heat  is  the  amount  of  heat  required  to  raise  a 
cubic  centimetre  of  water  one  degree  centigrade. 

To  raise  a  given  weight  of  mercury  a  certain  amount  in  temperature 
takes  only  one  thirty-third  of  the  heat  required  to  raise  an  equal  weight 
of  water  the  same  amount.  The  specific  heat  of  water  being  unity, 
that  of  mercury  is  0.03.  Mercury  at  32°  weighs  13.59593  times  as 
much  as  an  equal  volume  of  water  at  the  temperature  of  maximum 
density,  39°.  By  means  of  these  figures,  reducing  the  rate  at  which 
heat  is  received  by  the  mercury  of  the  thermometer-bulb  from  the  sun 
to  what  it  would  be  if  the  bulb  contained  water  instead  of  mercury,  and 
then  dividing  by  the  number  of  square  centimetres  in  the  cross-section 
of  the  bulb,  the  number  of  heat  units  is  obtained,  received  from  the  sun 
in  a  minute  on  a  surface  of  a  square  centimetre  perpendicular  to  the 
sun's  rays. 

The  amount  of  heat  received  in  a  minute  on  a  surface  of  one  square 
centimetre  vertical  to  the  sun's  rays  varies  with  the  clearness  of  the 
air,  and  the  height  in  the  air  at  which  the  experiments  are  made. 
When  suitable  allowance  is  made  for  the  amount  of  heat  absorbed  by 
the  thickness  of  the  air  between  the  place  of  observation  and  the 
upper  limit  of  the  air,  the  number  representing  the  solar-constant  is 
obtained,  which  is  about  3.0  gramme-calories. 


28 


METEOROLOGY. 


O 

FIG.  5. 


BAROMETER. 

Air  Pressure.  —  The  air  presses  on  everything  at  the  sur- 
face of  the  earth  with  a  pressure  equal  to  the  weight  of  the 
column  of  air  above  it  to  the  limit  of  the  atmosphere.  Pres- 
sure is  only  noticeable  to  the  senses  in  the  case  of  sudden 
and  very  great  changes,  as  in  a  balloon  ascent  or  in  ascend- 
ing a  mountain  where  there  is  a  rapid  decrease  of  pressure 
with  the  increase  of  height.  The  air  pressures  equal  to  two 
or  three  atmospheres  experienced  in  the  pneumatic  chambers 
of  bridge-caissons  produce  marked  and  peculiar  sensations. 
Under  a  pressure  of  two  atmospheres  plants  will  not  germi- 
nate and  quickly  die. 

Barometer.  —  The  barometer  is  an  instrument  for  measur- 
ing the  pressure  of  the  air.  The  pressure  is  always  expressed 
in  an  equivalent  height  of  a  column  of  mercury  reduced  to 
the  temperature  of  freezing-point.  The  ordinary  barometer 
shown  in  Fig.  5  is  a  glass  tube  closed  at  one  end  about  38 
inches  long  and  a  quarter  of  an  inch  in  internal  diameter, 
enclosed  in  a  brass  sheath,  with  the  open  end  dipping  into  a 
vessel,  called  the  cistern,  containing  mercury  which  extends 
inside  the  tube  up  to  a  height  corresponding  to  the  pressure 
of  the  air.  The  vertical  height  of  the  top  of  column  above 
the  mercury  in  cistern  is  called  the  barometer  reading.  The 
instrument  is  suspended  by  the  ring  at  A. 

The  height  of  the  top  of  the  column  of  mercury  is  per- 
petually varying  with  the  changing  pressure  of  the  air.  The 
barometer  is  set  up  by  first  filling  the  tube  with  mercury 
and  inverting  it  with  the  open  end  immersed  in  the  mer- 
cury in  cistern,  the  end  being  closed  temporarily  with  the 
gloved  finger  until  immersed  in  the  mercury.  The  mercury 
runs  down  the  tube,  when  the  finger  is  removed,  oscillating 
back  and  forth  until  the  column  is  of  the  right  height  to 
counterbalance  the  pressure  of  the  air  outside.  In  the  tube 
above  the  mercury  there  is  no  air.  This  space  is  called  the 
Torricellian  vacuum. 


METEOROLOGICAL  INSTRUMENTS.  29 

Filling  Tube.  —  A  tube  is  filled  by  pouring  three  or  four  inches  of 
mercury  into  the  glass  tube  and  boiling  it  in  the  tube  so  as  to  expel 
all  the  air  adhering  to  the  sides.  The  tube  and  mercury  are  heated 
slightly  before  the  mercury  is  poured  in.  The  tube  is  heated  gradually 
by  passing  it  to  and  fro  through  a  flame.  When  the  mercury  boils  and 
the  tube  is  bright  and  free  of  all  tarnish,  three  or  four  more  inches  of 
mercury  are  added  after  the  tube  has  cooled  down  some.  This  is 
boiled  as  before,  and  so  on,  three  or  four  inches  at  a  time,  until  the 
whole  tube  is  filled.  The  whole  tube  is  not  boiled  at  each  addition 
of  mercury,  but  merely  the  new  portion  added.  An  alcohol  lamp  with 
a  broad  flame  is  used  in  the  boiling  or  a  charcoal  furnace.  A  good 
substitute  for  a  lamp  is  a  shallow  earthenware  dish  holding  salt  or 
sand  saturated  with  alcohol. 

In  filling  barometer  tubes  pure  mercury  is  used,  prepared  by  distilla- 
tion of  commercial  mercury  in  a  vacuum.  Numerous  tubes  are  broken 
in  this  process  of  filling,  and  it  is  well  to  provide  some  means  of  saving 
the  mercury  in  case  a  tube  does  break.  The  boiling  should  be  done 
in  the  open  air  or  at  least  in  a  room  with  free  ventilation,  otherwise  the 
vapour  from  the  mercury  may  cause  salivation. 

Only  small  tubes  can  be  filled  successfully  in  the  way  described. 
Large  tubes,  one  inch  in  diameter,  are  almost  certain  to  break  if 
treated  in  this  way.  Even  if  they  do  not  break  in  the  process  of  boil- 
ing, they  are  sure  to  crack  from  flaws  developing  a  few  days  or  a  week 
after. 

A  plan  sometimes  followed  in  filling  tubes  is  to  exhaust  them  of  air 
in  connection  with  a  mercury-distilling  apparatus,  and  allow  the  filling 
to  take  place  gradually  as  the  mercury  distils  over. 

Measuring  Pressure. —  Measuring  the  pressure  of  the  air  by  means 
of  a  barometer  consists  in  ascertaining  the  vertical  distance  from  the 
level  of  mercury  in  the  cistern  to  the  top  of  the  column  in  the  tube. 
As  the  pressure  of  the  air  changes,  mercury  is  passing  in  or  out  of  the 
tube  and  constantly  changing  the  level  of  the  mercury  in  the  cistern. 
Before  an  observation  of  pressure  is  made,  the  level  of  the  mercury  in 
the  cistern  is  brought  to  the  ivory  point  shown  at  B,  in  the  figure. 
This  is  done  by  means  of  the  screw  O  at  the  bottom  acting  on  the 
leather  bag  of  the  cistern. 


METEOROLOGY. 


The  brass  tube  surrounding  the  glass  tube  carries  a  scale  graduated 
to  inches  and  tenths.  The  zero  of  the  graduation  is  the  ivory  point  in 
the  cistern. 

Vernier.  —  The  reading  of  the  scale  at  the  top  of  the  column  of 
mercury  is  made  by  means  of  a  vernier,  as  shown  in  Figs.  6,  7,  and  8. 

The  vernier  consists  of  a  short  tube  sliding  inside  of  the  brass  tube, 
and  accurately  adjustable  to  any  position  by  a  rachet  motion  worked 


30 


_Pr 


30 


10 


FIG.  6. 


FIG.  7. 


FIG.  8. 


by  a  milled-head  screw  on  the  side  of  tube.  In  observing  a  pressure, 
the  lower  edge  of  the  vernier  is  brought  tangent  to  the  top  of  the 
rounding  end  of  the  column  of  mercury,  which  is  called  the  meniscus. 
A  space  of  nine-tenths  of  an  inch  on  the  vernier,  from  the  bottom,  is 
graduated  to  ten  equal  parts.  Each  interval  is  therefore  about  0.09 
of  an  inch. 

If  the  first  mark  above  the  bottom  of  the  vernier  coincides  with  a 
scale  division,  the  intervals  on  the  barometer  scale  being  o.  10  of  an 
inch,  it  shows  that  the  top  of  the  column  of  mercury  is  o.oi  of  an  inch 
above  the  even  division  on  scale.  If  the  second  mark  corresponds  with 
a  division  on  scale,  it  shows  that  the  top  of  column  is  0.02  of  an  inch 


METEOROLOGICAL  INSTRUMENTS.  31 

above  an  even  division,  and  so  on  for  the  third,  fourth,  fifth,  etc., 
marks  on  the  vernier. 

When  there  is  not  an  exact  correspondence  between  any  particular 
mark  and  division,  then  two  marks  of  the  vernier  are  inside  of  two 
division  marks  on  the  scale,  and  the  reading  is  to  be  taken  as  half-way 
between  the  two  one-hundredth  marks,  or  to  the  nearest  half-hundredth 
or  0.005  °f  an  incn-  The  verniers  in  the  figures  read  30.000,  29.250, 
and  30.055  inches.  The  limit  of  accuracy  of  pressures  obtainable  with 
a  portable  barometer  is  about  0.005  °f  an  mcn- 

Suspension.  —  The  barometer  when  in  use  is  suspended  by  a  ring  at 
the  top  of  the  brass  tube  and  allowed  to  hang  freely.  The  scale  of  the 
barometer  is  set,  on  leaving  the  marker's  hands,  so  as  to  read  the  same 
as  a  standard  barometer.  In  small  tubes,  only  a  quarter  of  an  inch  in 
diameter,  the  capillary  action  between  the  glass  and  the  mercury  de- 
presses the  surface  of  the  mercury  0.04  of  an  inch.  Setting  the  instru- 
ment by  reference  to  a  standard  is  equivalent  to  allowing  for  capillarity. 
The  depression  is  greater  the  less  the  diameter  of  the  tube.  It  is  inap- 
preciable (less  than  0.0004  °f  an  inch)  for  a  tube  1.2  inch  in  diameter. 

Repairs  or  alterations  to  the  suspending  ring  of  a  barometer  or  the 
addition  of  any  unsymmetrically  disposed  weight,  as  for  instance  a  new 
and  heavier  attached  thermometer  after  the  instrument  is  standardized, 
may  cause  the  axis  of  the  barometer  to  hang  in  a  slightly  different  posi- 
tion from  what  it  did  before.  A  displacement  of  the  axis  equal  to  three 
degrees  in  a  direction  from  the  centre  of  the  tube  toward  the  ivory 
point  may  make  a  difference  of  0.04  of  an  inch  in  the  reading,  depend- 
ing on  the  horizontal  distance  from  the  centre  of  the  tube  to  the  ivory 
point.  This  is  a  common  source  of  error  in  barometers. 

Standard  Barometer.  —  A  standard  barometer  is  the  same  in  principle 
as  the  instrument  described.  The  tube  is  heavier,  —  half  an  inch  or  one 
inch  or  more  in  diameter.  For  measuring  the  differences  in  level  of 
the  surfaces  of  mercury  a  cathetometer  is  used,  which  consists  of  a  pair 
of  microscopes  moving  up  and  down  on  a  steel  bar.  When  the  cross- 
wires  of  microscopes  are  adjusted  to  the  heights  of  the  mercurial  sur- 
faces, the  steel  bar  is  revolved  so  as  to  transfer  the  distance  to  a 
graduated  scale.  Great  precautions  are  taken  to  have  pure  mercury  in 
a  standard  barometer,  and  to  have  all  air  excluded  from  the  space  above 


32  METEOROLOGY. 

the  column  of  mercury.  A  number  of  tubes,  if  at  least  half  an  inch  in 
inside  diameter,  will,  on  careful  comparison,  without  any  standardizing, 
give  pressures  agreeing  within  0.005  of  an  inch. 

Reduction  for  Temperature.  —  Barometers  observed  at  different  tem- 
peratures require  reduction  to  a  common  temperature  before  the  obser- 
vations are  strictly  comparable  as  pressures.  The  temperature  selected 
is  the  freezing-point.  An  attached  thermometer  at  the  middle  of  the 
barometer  gives  its  temperature. 

As  the  cubical  expansion  of  mercury  is  about  ten  times  as  great  as 
that  of  brass,  the  reduction  for  temperature  is  minus  at  ordinary  temper- 
atures, or  the  barometer  readings  are  too  high.  The  reduction  is  minus 
down  to  28°,  where  it  changes,  for  lower  temperatures,  to  plus.  The 
reason  the  turning-point  is  not  at  32°  is  because  the  scale-length  is 
reduced  to  a  temperature  of  62°,  for  which  measures  of  length  are 
standard,  and  the  mercury  to  freezing-point,  the  whole  correction  to  the 
reading  being  the  difference  of  the  two  corrections. 

Place  for  Barometer.  —  A  barometer  should  be  kept  in  a  room  of 
nearly  uniform  temperature.  In  heated  rooms  in  winter  there  is  often 
a  difference  of  ten  degrees  or  more  between  the  top  and  bottom  of  a 
barometer.  The  attached  thermometer  in  such  a  case  does  not  give  the 
average  temperature  of  the  barometer  accurately.  An  error  of  one 
degree  in  the  temperature  introduces  an  error  of  0.003  of  an  mch  in 
the  derived  pressure. 

Correction  for  Height.  —  As  the  pressure  of  the  air  varies  with  height, 
the  observed  pressures  at  different  places,  to  be  comparable,  must  be 
reduced  to  a  common  level.  The  level  of  the  sea  is  usually  selected  for 
this  purpose.  For  a  height  of  1000  feet  above  sea  level  the  reduction 
when  the  temperature  of  the  air  is  60°  is  1.06  inches  to  be  added ; 
at  30°,  1.12. 

Gravity  Correction.  —  As  the  intensity  of  gravity  varies  with  latitude 
and  height  above  sea  level,  the  same  pressures  are  represented  by 
slightly  different  lengths  of  columns  of  mercury  in  different  places. 
The  reduction  to  latitude  45°  and  sea  level  is  called  the  gravity  cor- 
rection. For  a  pressure  of  30  inches,  if  observed  at  the  equator,  the 
corection  is  0.078  of  an  inch  to  be  subtracted ;  at  the  pole  the  same 
amount  is  additive.  For  a  height  of  one  mile  above  sea  level,  where 


METEOROLOGICAL  INSTRUMENTS. 


33 


the  pressure  is  25  inches,  the  reduction  is  about  0.012  of  an  inch  to  be 
subtracted. 

Aneroid  Barometer.  —  An  aneroid  barometer,  shown  in  Fig.  9,  con- 
sists of  a  metal  box  from  which  the  air  is  exhausted,  and  a  steel 
spring  in  the  form  of  a  folded  leaf.  The  corrugated  top  of  the  box,  of 
very  thin  and  yielding  metal,  is  fastened  to  the  upper  side  of  the  spring. 
The  bottom  of  the  box  is  fastened  to  the  lower  end.  The  top  of  the 


FIG.  9. 


box  exposes  three  or  four  square  inches  of  surface  to  the  air.  The 
pressure  of  the  atmosphere  on  this  is  from  40  to  60  pounds.  It  is  sus- 
tained by  the  spring,  which  yields  as  the  weight  varies  with  the  chang- 
ing pressure  of  the  air.  A  system  of  levers  transfers  and  magnifies  the 
motion  of  the  end  of  the  spring  to  an  index  which  moves  around  a  dial. 

Compensation  for  Temperature.  —  Aneroids  are  graduated  by  refer- 
ence to  a  mercurial  barometer. 

The  elasticity  of  springs  varies  with  temperature.  A  given  weight 
will  pull  down  a  spring  more  at  a  high  temperature  than  at  a  low  one. 
In  the  best  forms  of  aneroids  an  adjustment  for  temperature  is  intro- 
duced by  making  one  of  the  transmitting  levers  a  compound  metal  of 


34  METEOROLOGY. 

steel  and  brass.  The  compensation  for  temperature  is  only  rarely 
accomplished  with  accuracy,  and  is  a  mere  accident  when  it  does 
occur.  A  change  of  twenty-five  degrees  will  often  cause  the  pressure 
indication  to  vary  o.io  of  an  inch  without  any  actual  change  of  pressure, 
in  some  instruments  one  way  and  in  others  in  the  opposite  way. 

A  compensation  is  sometimes  attempted  by  leaving  some  air  in  the 
metal  box.  As  the  spring  weakens  at  a  high  temperature,  the  increased 
pressure  of  the  air  inside  the  box  counterbalances  it  to  some  extent. 
This  is  ineffectual  as  a  compensation. 

The  tension  of  a  spring  not  varying  with  latitude,  aneroid  barometers 
need  no  gravity  correction. 

The  aneroid  is  not  as  good  an  instrument  for  indicating  pressures  as 
a  mercurial  barometer.  Changes  take  place  in  the  nature  of  the  spring 
with  age  and  extreme  variations  of  pressure,  which  cause  its  indications 
to  vary  slightly,  independently  of  variations  in  the  pressure  of  the  air. 
In  three  years  the  pressure  indication  of  an  aneroid  will  increase  0.08 
of  an  inch.  It  requires  constant  correction  and  control  by  reference 
to  a  standard  mercurial  barometer.  After  subjection  to  a  low  pressure, 
as  in  a  mountain  ascent,  an  aneroid  does  not  recover  its  original  reading 
at  once  for  ordinary  pressures.  After  lowering  to  a  pressure  of  18 
inches,  its  reading  will  be  0.3  of  an  inch  lower  when  brought  back  to 
a  pressure  of  30  inches  than  it  was  before.  The  original  reading  is, 
however,  recovered  in  a  few  weeks.  There  is  a  slow  progressive  rise 
with  age  in  the  pressures  indicated  by  an  aneroid.  The  instrument, 
however,  on  account  of  its  convenience,  is  used  a  good  deal  in  rough 
determinations  of  altitudes.  It  is  rarely  used  in  meteorological  obser- 
vations. 

The  words  "fair,"  "stormy,"  "rain,"  etc.,  sometimes  to  be  seen  on 
aneroids,  have  no  real  significance  as  regards  the  weather,  being  a 
device  intended  to  help  the  sale  of  the  instruments. 

Hypsometer.  —  The  boiling-point  of  water  is  lower  in  temperature 
the  less  the  pressure  of  the  air.  While  the  boiling-point  is  22i°.oat 
a  pressure  of  29.922  inches,  it  is  only  187°.  5  at  a  pressure  of  18  inches. 
This  fact  is  sometimes  made  use  of  in  determining  pressure,  especially 
in  the  measurement  of  altitudes  in  mountain  ascents.  A  thermometer 
with  its  boiling-point  apparatus,  when  used  for  this  purpose,  is  called 


METEOROLOGICAL  INSTRUMENTS.  35 

a  hypsometer.  Thermometers  for  this  purpose  are  made  short  for 
convenience  in  carrying,  and  are  only  graduated  from  about  160°  to 
214°.  Such  a  thermometer  should  also  have  some  graduation  in  the 
vicinity  of  the  freezing-point.  An  enlargement  of  the  tube  between 
the  freezing-point  and  the  graduation  of  160°  will  permit  of  this  with- 
out having  the  thermometer  stem  of  an  inconvenient  length.  After 
an  observation,  due  allowance  for  the  change  of  freezing-point  of  the 
instrument  will  greatly  improve  the  accuracy  of  the  temperatures 
observed,  and  consequently  also  of  the  pressures  derived. 

Vapour  Pressure. — Vapour  of  water  exerts  a  pressure  in  the  air.  Like 
the  other  constituents,  it  is  transparent  and  only  becomes  visible  on 
changing  to  fog  or  cloud,  which  is  water  in  a  very  fine  state  of  division, 
the  particles  varying  in  diameter  from  0.0006  to  0.0050  of  an  inch. 

Saturation. — The  quantity  of  moisture  that  can  exist  as  vapour  in  the 
air  depends  on  its  temperature.  There  is  a  certain  pressure  of  vapour 
corresponding  to  every  temperature,  which  cannot  be  exceeded.  This 
is  called  the  pressure  of  saturation  for  the  temperature.  If  the  tem- 
perature is  diminished,  part  of  the  vapour  is  thrown  down  as  fog,  rain, 
or  dew.  The  temperature  at  which  this  begins  is  called  the  dew-point. 
This  property  of  vapour  is  made  use  of  in  determining  the  pressure  of 
vapour  in  the  air.  The  number  of  degrees  the  dew-point  is  below  the 
air  temperature  is  called  the  depression  of  the  dew-point.  The  depend- 
ence of  vapour  pressure  and  temperature  has  been  thoroughly  investi- 
gated by  skilful  experimenters,  on  account  of  its  importance  in  the 
theory  of  the  steam  engine. 

The  saturation  pressure  in  inches,  and  weight  of  vapour  in  a  cubic 
foot  of  air  in  grains,  at  temperatures  ten  degrees  apart,  from  o°  to 
1 00°  F.,  are  as  follows  :  — 

Temperature,  o°  10°  20°  30°  40°  50°  60°  70°  80°  90°  100° 
Pressure,  inches,  0.045  0-071  o.uo  0.166  0.246  0.360  0.517  0.732  1.022  1.408  1.916 
Weight,  grains,  0.54  0.84  1.30  1.97  2.86  4.09  5.76  7.99  10.95  14.81  19.79 

The  quantity  of  water  in  the  air  is  nearly  proportional  to  the  vapour 
pressure.  The  air  is  rarely  perfectly  saturated,  not  always  even  when 
rain  is  falling.  Neither  is  the  air  ever  perfectly  dry  at  any  place. 

Relative  Humidity.  —  The  vapour  pressure  actually  prevailing  in  the 
air,  divided  by  the  vapour  pressure  at  saturation  for  the  temperature 


36  METEOROLOGY. 

of  the  air,  is  the  relative  humidity.  It  is  expressed  in  percentage  of 
saturation.  For  perfectly  dry  air,  the  relative  humidity  would  be  zero ; 
for  saturation,  one  hundred.  Relative  humidity  only  expresses  relative 
amounts  of  moisture  in  the  air  for  the  same  temperatures.  At  a  low 
temperature,  a  high  relative  humidity  represents  a  very  small  actual 
amount  of  vapour  in  the  air,  while  a  low  relative  humidity  at  a  high 
temperature  may  represent  a  great  quantity.  For  instance,  a  relative 
humidity  of  90  at  a  temperature  of  30°  corresponds  to  a  vapour  pressure 
of  0.149  of  an  inch,  while  50  per  cent  at  80°  corresponds  to  a  pressure 
of  0.51 1,  or  more  than  three  times  as  much.  Relative  humidities  have, 
therefore,  no  significance  as  representing  actual  quantities  of  moisture 
in  the  air  at  different  times  and  places. 

Dew-point  Apparatus.  —  A  dew-point  apparatus  consists  of  a  thin 
brass  tube,  one  inch  in  diameter  and  six  inches  long,  silvered  or  gilt 
on  the  outside.  A  thermometer  fitted  in  a  cork  or  rubber  stopper  has 
its  bulb  inside  the  tube,  with  the  stem  projecting.  A  quantity  of  some 
volatile  liquid,  as  rhigolene  or  ether,  is  contained  in  the  tube  so  as  to 
cover  the  thermometer  bulb.  Openings  in  the  cork  contain  tubes,  one 
of  which  dips  into  the  liquid,  and  the  other  has  its  end  some  distance 
above  the  surface  of  the  liquid.  By  means  of  these  tubes,  a  current  of 
air  is  blown  or  aspirated  through  the  liquid,  and  evaporation  induced, 
which  lowers  the  temperature  of  the  liquid  and  the  tube  containing  it. 
When  the  temperature  of  the  dew-point  is  reached,  a  film  of  moisture 
forms  on  the  polished  surface  of  the  tube.  The  temperature  then 
shown  by  the  thermometer  is  that  of  the  dew-point  of  the  air. 

When  the  temperature  of  the  dew-point  is  very  much  below  that  of 
the  air,  there  has  to  be  a  lowering  of  the  temperature  of  the  liquid 
considerably  below  the  dew-point  in  order  to  make  an  appreciable 
deposit  of  dew.  For  this  reason,  the  temperature  given  by  the  ther- 
mometer is  usually  slightly  lower  than  that  of  the  true  dew-point.  As 
soon  as  the  film  of  moisture  is  seen  to  form,  the  aspiration  or  blowing 
is  stopped.  Immediately  the  temperature  begins  to  rise,  and  soon 
the  film  disappears.  The  mean  of  the  temperatures  at  the  times  of 
appearance  and  disappearance  of  the  film  is  usually  taken  as  the  true 
temperature  of  dew-point.  These  may  differ  as  much  as  0.6  of  a 
degree. 


METEOROLOGICAL  INSTRUMENTS.  37 

The  dew-point  being  known,  the  vapour  pressure  in  the  air  can  be 
found  from  suitable  tables. 

The  dew-point  apparatus  is  not  used  extensively  in  the  regular  work 
of  determining  vapour  pressures,  on  account  of  the  inconvenience  of 
its  use  and  the  expense  in  the  use  of  liquids.  It  is  only  used  in  the 
fundamental  operation  of  standardizing  psychrometers.  For  small 
depressions  of  dew-point,  at  ordinary  temperatures,  ammonia  water 
can  be  used  with  brisk  aspiration  to  produce  a  lowering  of  tempera- 
ture. A  brass  tube,  however,  will  be  corroded  by  ammonia. 

Psychrometer.  —  Evaporation  from  a  water  surface  into  the  air  takes  f 
place  to  a  greater  or  less  amount,  depending  on  the  dryness  of  the  air, 
the  rapidity  with  which  the  air  over  the  surface  is  renewed,  and  upon 
the  temperature  of  the  water.  Evaporation  produces  cooling.  This 
property  is  taken  advantage  of  to  determine  the  vapour  pressure  in  the 
air.  A  mercurial  thermometer  with  a  wrapping  of  muslin  on  its  bulb, 
kept  saturated  with  water,  when  read  in  connection  with  an  ordinary 
dry  thermometer,  can  be  used  for  determining  the  pressure  of  the 
vapour  in  the  air,  and  is  then  called  a  psychrometer.  Water  evap- 
orating from  the  muslin  cools  the  bulb.  The  dryer  the  air,  the  greater 
the  evaporation  and  the  greater  the  amount  of  cooling. 

From  a  comparison  of  the  indications  of  a  psychrometer  and  a  dew- 
point  apparatus  at  the  same  time,  it  is  ascertained  that  the  vapour- 
pressure  at  the  dew-point  of  the  air  is  equal  to  the  vapour  pressure 
corresponding  to  the  temperature  of  the  wet  bulb  thermometer,  minus 
the  number  o.oii  multiplied  by  the  difference  in  degrees  between  the 
dry  and  wet  bulb  thermometers.  This  applies  for  a  pressure  of  the  air 
equal  to  30  inches,  and  when  the  psychrometer  is  whirled.  The  quan- 
tity to  be  subtracted  increases  slightly  with  the  pressure,  and  also  with 
the  difference  between  the  dry  and  wet  bulb. 

The  results  for  vapour  pressure,  obtained  with  a  psychrometer,  differ 
a  little  in  still  air,  according  as  the  bulb  of  the  thermometer  is  spherical 
or  cylindrical ;  they  also  depend  on  the  absolute  size  of  the  bulb,  and 
whether  the  stem  is  large  or  small,  and  whether  or  not  the  muslin  is 
wrapped  about  the  stem  for  some  distance  above  the  bulb,  so  as  to  cool 
it  as  well  as  the  bulb  and  prevent  conduction  of  heat  from  the  stem  to 
the  bulb.  All  of  these  effects  are  obviated,  and  instruments  of  the 


38  METEOROLOGY. 

most  diverse  shapes  become  comparable,  when  the  psychrometer  is 
whirled  before  readings  are  made,  so  as  to  keep  up  a  brisk  renewal 
of  air  around  the  bulb. 

Where  the  greatest  accuracy  is  required,  the  most  minute  corrections 
of  the  thermometers  have  to  be  applied.  Small  errors  in  the  tempera- 
tures affect  the  deduced  vapour  pressures  very  much,  especially  at  low 
temperatures.  The  thermometer  readings  should  be  corrected  for  the 
difference  of  temperature  between  the  bulb  and  stem.  No  existing 
psychrometer  tables,  however,  are  constructed  on  that  plan. 

In  the  case  of  a  still  psychrometer  about  freezing-point  the  wet  bulb, 
or  bulb  with  a  coating  of  ice  on  the  muslin,  is  apt  to  read  half  a  degree 
higher  than  the  dry  bulb.  This  difference  is  done  away  with  also  by 
whirling. 

When  there  is  a  very  thick  coating  of  ice  on  the  bulb,  the  contrac- 
tion being  greater  than  that  of  glass,  there  is  an  effect  of  compression 
which  may  cause  an  ice-covered  bulb  to  read  several  tenths  of  a  degree 
too  high.  This  does  not  occur  unless  the  ice  around  the  bulb  is  heavy 
and  continuous. 

The  vapour  from  ice  at  a  temperature  of  32°  is  of  less  pressure  than 
from  water  of  the  same  temperature. 

Hair  Hygrometer.  —  A  human  hair,  when  freed  of  oil  by  soaking  in 
ether  for  twenty-four  hours,  has  the  property  of  changing  its  length  by 
about  one-thirtieth  part  between  a  very  dry  and  very  moist  condition  of 
the  air  at  ordinary  temperatures.  A  length  of  about  12  inches  is 
usually  mounted  in  a  light  brass  frame,  the  lower  end  wound  once 
around  a  pulley  and  stretched  by  a  weight  of  about  one  gramme.  As 
it  varies  in  length,  the  pulley  turns,  moving  an  index  along  a  scale 
graduated  to  represent  relative  humidities.  It  is  graduated  by  reference 
to  a  dew-point  apparatus  or  a  psychrometer.  The  hair  hygrometer  is 
not  an  accurate  instrument,  its  indications  being  complicated  with  tem- 
perature. 

Rainfall. — A  rainfall  is  measured  as  the  depth  of  water  it  would  form 
on  the  ground  were  it  all  to  remain  as  it  falls.  In  the  case  of  snow, 
its  depth  is  the  depth  of  water  it  would  form  if  melted.  Rainfall, 
snowfall,  and  hail  are  known  by  the  general  designation  of  pre- 
cipitation. It  is  always  preferable  to  measure  snowfall  as  depth  of 


METEOROLOGICAL  INSTRUMENTS. 


39 


"melted  snow"  rather  than  only  as  depth  of  snow,  but  if  possible 
both  should  be  used.  The  specific  relation  between  depth  of  snow 
and  depth  of  "melted  snow"  varies  from  \  to  ^  in  different  cases,  de- 
pending on  the  saturation  of  the  snow.  The  factor  -fa  is,  however, 
commonly  used  to  reduce  depth  of  snow  to  depth  of  melted  snow  when 
it  cannot  be  melted  for  measurement. 

Rain  Gauge.  —  For  the  measurement  of  rain  a  rain  gauge,  shown  in 
Fig.  10,  is  used.  It  consists  of  three  parts.  The  top  is  a  funnel-shaped 
piece  of  galvanized  iron  with  a  verti- 
cal rim  of  brass,  its  inside  diameter 
being  8  inches.  It  fits  into  a  cylinder 
of  galvanized  iron  8  inches  in  diam- 
eter and  20  inches  deep.  Inside  of 
this  is  a  brass  cylinder  20  inches 
long  and  the  area  of  its  interior 
cross-section  just  one-tenth  that  of 
the  brass  rim  or  funnel.  All  the  rain 
falling  over  a  surface  equal  to  the 
section  of  the  rim  is  gathered  and 
runs  into  the  brass  tube.  The  depth 
of  water  in  the  tube  is  ten  times  the 
depth  of  the  rainfall. 

The  depth  of  water  is  measured  by 
dipping  into  it  a  thin  cedar  stick 
graduated  to  inches  and  tenths.  On 

withdrawing  it,  the  depth  of  water  can  be  seen  on  the  wetted  part  of 
the  stick. 

When  the  rainfall  exceeds  two  inches,  it  runs  out  of  the  tube  into  the 
galvanized  iron  cylinder  called  the  overflow. 

Snow  Gauge.  —  A  snow  gauge  is  simply  a  galvanized  iron  cylinder 
8  inches  in  diameter.  Usually  a  rain  gauge  is  used  in  measuring 
snow  which  is  melted  before  measuring. 

The  depth  of  rainfall  collected  by  different  rain  gauges  varies  slightly 
with  the  size  of  the  gauge.  The  larger  the  size  the  better,  but  there 
is  very  little  advantage  gained  by  a  size  greater  than  8  inches. 
Funnels  of  4  inches  in  diameter  are  too  small,  and  do  not  give  as 
great  a  depth  of  rain  as  larger  ones. 


FIG.  10. 


4<D  METEOROLOGY. 

Rain-gauge  Exposure.  —  Rain  gauges  in  slightly  different  positions 
differ  greatly  in  the  depth  of  rain  indicated.  Within  a  few  yards  of 
each  other,  the  depth  in  a  single  rain-storm  will  sometimes  differ  20 
per  cent,  as  shown  by  two  gauges.  The  stronger  the  wind,  usually, 
the  greater  the  differences  between  gauges.  In  an  exposed  situation 
a  wind  creates  eddies  which  divert  the  rain  that  would  otherwise  fall 
into  the  gauge.  A  gauge  near  the  edge  of  a  roof,  on  the  windward 
side  of  a  building,  shows  a  markedly  less  rainfall  than  one  in  the  centre 
of  the  roof.  This  is  due  to  an  ascending  current  vertically  along  the 
wall,  spreading  slightly  over  the  edge  and  carrying  away  some  part  of 
the  rain. 

Forests  intercept  from  6  to  1 5  per  cent  of  the  rainfall  by  the  leaves 
and  branches  of  the  trees. 

A  rain  gauge  at  a  height  of  43  feet  collects  only  0.75  as  much 
rainfall  as  one  at  the  ground;  at  85  feet,  0.64;  and  at  194  feet,  0.58. 
These  differences  are  purely  the  effects  of  wind  currents. 

A  fence  three  feet  high  around  a  gauge  at  a  distance  of  three  feet 
will  cause  it  to  collect  six  per  cent  more  rain  than  when  unprotected. 

Percolation  Gauge  or  Lysimeter.  —  An  instrument  for  measuring  the 
amount  of  rainfall  that  reaches  different  depths  in  the  earth  is  called 
a  percolation  gauge  or  a  lysimeter.  It  consists  of  an  iron  vessel  three 
feet  in  diameter  and  three  feet  deep,  filled  with  earth,  and  imbedded 
in  the  earth  with  its  top  surface  level  with  the  ground,  and  means 
provided  for  drawing  off  the  water  that  collects  at  the  bottom.  The 
amount  of  rainfall  that  reaches  a  depth  of  three  feet  is  about  one-third 
or  one-fourth  of  the  annual  rainfall  at  the  surface  of  the  ground ;  it 
varies  with  the  nature  of  the  soil.  The  maximum  occurs  at  a  different 
time  of  year  from  that  at  the  surface. 

Anemometer.  —  The  velocity  of  wind  is  usually  stated  as  the  number 
of  miles  per  hour  travelled.  Sometimes  the  intensity  of  wind  is  stated 
as  the  pressure  exerted  in  pounds  on  a  surface  of  a  square  foot.  An 
anemometer  is  used  for  measuring  the  velocity  of  the  wind.  The  form 
known  as  Robinson's  anemometer  is  shown  in  Fig.  1 1,  and  is  the  form 
in  general  use. 

The  common  form,  the  pattern  used  in  the  United  States  Weather 
Bureau,  has  hemispherical  cups  four  inches  in  diameter  on  cross-arms, 


METEOROLOGICAL  INSTRUMENTS. 


the  centres  of  the  cups  6.72  inches  from  the  intersection  of  the  cross- 
arms  or  centre  around  which  they  revolve.  The  cross-arms  are  fastened 
to  an  axis  or  spindle,  free  to  rotate.  A  screw  on  the  lower  end  of  axis 
works  a  system  of  dials,  so  arranged  as  to  register  approximately  in 


FIG.  ii. 

miles  by  the  rotation  of  the  cups  the  number  of  miles  travelled  by  the 
wind.  The  difference  in  the  pressure  of  the  wind  on  the  rounded  and 
hollow  sides  of  the  cups  at  the  ends  of  the  arms  cause  the  cross-arms 
and  axis  to  rotate.  The  pressure  on  the  hollow  sides  is  greater  than 
on  the  rounded.  The  dials  are  geared  to  register  a  mile  for  every  five 
hundred  turns  of  the  spindle.  This  is  equivalent  to  assuming  that  the 
wind  travels  three  times  as  fast  as  the  centre  of  the  cups. 

Corrections  of  Anemometers.  —  As  a  matter  of  fact  the  ratio  between 
the  wind  and  cup  velocity  varies  with  the  size  and  inertia  of  the  cups, 
the  length  of  the  arms,  and  the  greater  or  less  velocity  and  irregularity 
of  the  wind.  At  high  velocities  the  ratio  is  less  than  at  low  velocities. 
In  the  form  described  the  registered  and  true  velocities  have  the  rela- 


42  METEOROLOGY. 

tion  given  below,  as  shown  by  Marvin's  experiments.  The  average 
ratio  for  ordinary  velocities  is  2.73  instead  of  3.00,  as  is  assumed  in 
the  dialing  of  the  instrument  to  indicate  miles  travelled  by  the  wind. 

Registered  velocity,  miles  per  hour,  10     20     30     40     50     60     70     80     90 
True  velocity,  miles  per  hour,  9.6  17.8  25.7  33.3  40.8  48.0  55.2  62.2  69.2 

These  results  were  ascertained  by  whirling  an  anemometer  at  differ- 
ent velocities,  attached  to  the  end  of  an  arm  32  feet  long,  and  comparing 
the  registered  with  the  actual  velocity,  known  by  counting  the  revolu- 
tions of  the  arm. 

Wind  Pressure.  —  The  wind  pressure  in  pounds  per  square  foot  on 
a  surface  is  equal  to  0.004  multiplied  by  the  square  of  the  velocity  in 
miles  per  hour. 

Beaufort  Scale.  —  The  velocity  of  wind  at  sea  is  estimated  on  a  scale 
of  numbers  from  i  to  12,  depending  on  the  amount  of  sail  a  ship  is 
capable  of  carrying  in  the  different  winds.  The  scale  in  common  use 
is  known  as  the  Beaufort  scale.  The  corresponding  velocities  in  miles 
per  hour  have  been  ascertained  by  comparison  of  estimates  of  the  wind 
on  sea  with  wind  velocities  observed  with  anemometers  on  shore  in  the 
vicinity  at  the  same  time.  The  estimates  on  the  scale  are  made,  for  the 
most  part,  according  to  the  commotion  the  wind  causes  in  the  water  or 
the  rigging  of  the  ship.  The  velocities  are,  of  course,  not  as  accurate 
as  those  obtained  with  an  anemometer. 

WIND  FORCE.  VELOCITY. 

Beaufort  Scale.  Miles  per  hour. 

o Calm o 

i Light  air 3 

2 Light  breeze 13 

3 Gentle  breeze 18 

4 Moderate  breeze 23 

5 Fresh  breeze 28 

6 Strong  breeze 34 

7 Moderate  gale 40 

8 Fresh  gale 48 

9 Strong  gale 56 

10 Whole  gale 65 

ii Storm 75 

12 Hurricane 90 


METEOROLOGICAL  INSTRUMENTS.  43 

The  above  velocities,  not  being  corrected  for  the  errors  of  anemom- 
eter, are  probably  too  large.  The  English  Meteorological  Service 
anemometers,  on  which  these  velocities  are  based,  are  of  a  larger 
pattern  than  those  in  use  in  the  United  States.  The  coefficient  for 
3<>mile  velocities  is  probably  2.40,  as  shown  by  Dohrandt's  experi- 
ments, instead  of  3.00,  for  which  the  English  instruments  are  geared 
and  dialed. 

Storm  Wind.  —  A  storm  wind  at  sea  is  50  miles  an  hour,  uncorrected 
velocity.  This  is  the  limit  adopted  by  the  English  Admiralty  Courts 
as  the  lowest  for  which  a  vessel  can  be  excused  on  account  of  stress 
of  weather  in  cases  of  non-fulfilment  of  contract  in  a  specified  time, 
as  a  common  carrier,  or  in  which  insurance  claims  for  damages  can 
be  allowed.  A  sea-worthy  vessel  is  presumed,  at  the  lowest,  to  be 
able  to  weather  a  wind  velocity  of  50  miles  an  hour. 

Wind  Vane.  —  The  wind  vane  for  observing  the  direction  of  the  wind 
is  the  oldest  form  of  meteorological  instrument,  being  in  use  at  Athens 
before  the  Christian  era.  It  consists  of  an  arrow  on  an  upright  rod, 
free  to  revolve  and  take  the  direction  of  the  wind.  The  common  form 
used  in  meteorological  observatories  is  six  feet  long.  The  back  half  is 
of  wood,  ten  inches  wide,  split  down  the  centre,  and  spread  to  an  angle 
of  ten  degrees.  This  diminishes  the  oscillation  of  the  vane  in  a  strong 
wind.  The  front  part  of  arrow  is  an  iron  rod  with  a  spear-head.  A 
metal  ball  slides  along  the  rod,  and  can  be  fastened  in  position  to  make 
the  arrow  balance  on  the  centre  around  which  it  revolves.  The  arrow 
takes  the  position  which  exposes  the  least  amount  of  surface  vertical  to 
the  direction  of  the  wind. 

In  speaking  of  the  direction  of  the  wind,  the  direction  from  which 
the  wind  comes  is  always  meant.  In  meteorological  observations  the 
direction  is  usually  taken  to  the  nearest  forty-five  degrees  on  the  circle, 
or  half-way  between  the  cardinal  points  of  the  compass, — north,  south, 
east,  and  west. 

Self -register.  —  In  connection  with  an  anemometer  a  self-register  is 
commonly  used,  which  saves  the  trouble  of  going  to  the  roof  of  a  build- 
ing to  consult  the  dials  of  the  instrument.  One  is  shown  in  Fig.  12. 
The  anemometer  self-register  consists  of  a  small  drum  4  inches  in  diam- 
eter and  4  inches  long,  holding  a  fillet  of  paper.  It  is  revolved  by 


44 


ME  TE  OROLOG  Y. 


clock-work.  -An  electric  circuit  extends  from  the  register  to  the  ane- 
mometer on  the  roof  of  building.  Every  mile  registered  by  the  anemom- 
eter completes  the  circuit  and  brings  down  an  armature  to  which  is 
attached  a  pencil,  marking  on  the  paper,  making  a  notch  in  the  continu- 


FlG.    12. 

ous  lead-pencil  line.  The  process  is  repeated  whenever  a  mile  is  com- 
pleted. By  counting  the  number  of  notches  for  a  certain  time,  the 
velocity  of  the  wind  can  be  derived. 

The  velocity  of  the  wind  is  usually  taken  as  the  rate  at  which  it  has 
been  travelling  for  five  minutes  before  the  time  of  an  observation.  If, 
for  instance,  in  the  five  minutes  preceding  eight  o'clock  in  the  morning 
the  wind  travel  was  5  miles,  the  velocity  was  60  miles  an  hour. 

Anemograph.  —  For  recording  the  direction  of  the  wind  a  device 
called  an  anemograph  is  used,  somewhat  similar  to  the  anemometer 
described  above  for  recording  wind  velocity. 

Pluviograph.  —  An  instrument  which  records  the  rainfall,  the  sim- 
plest working  by  the  rising  and  falling  of  a  float  attached  to  a  pencil- 
marking  on  a  sheet  of  paper,  is  called  a  pluviograph. 

Self-registering  Instruments.  —  Besides  the  forms  of  self-registering 


METEOROLOGICAL  INSTRUMENTS. 


45 


instruments  described,  there  are  various  other  forms  for  recording  tem- 
perature, pressure,  and  humidity,  both  electrically  and  by  photography, 
and  in  other  ways.  These  are  called  thermographs,  barographs,  etc. 
The  sheets  of  paper  containing  the  records  are  called  thermograms, 
barograms,  etc.  Instruments  of  this  sort  are  often  of  complex  con- 
struction, and  costly.  To  keep  them  in  working  order  requires  the 
constant  attention  of  a  skilled  mechanician.  Great  labour  is  required 
to  turn  their  indications  into  numbers,  before  the  results  are  of  any  use. 
Cloud  Motion.  —  A  nephoscope,  shown  in  Fig.  13,  is  an  instru- 
ment used  in  observing  cloud  motion.  One  form  of  this  instrument 
consists  of  a  brass  circle  with 
divisions  to  five  degrees.  In- 
side the  circle  is  a  mirror. 
Two  wires  at  right  angles, 
intersecting  at  the  centre, 
can  be  moved  in  any  position 
around  the  circle.  On  the 
side  of  the  circle  there  is  a 
metal  post  with  a  knob  on 


top.  The  post  is  hinged  so 
that  it  can  be  inclined  in  any 
position,  or  shut  down  when 
the  instrument  is  not  in  use. 
In  use  on  land,  the  instru- 
ment is  fixed  with  the  zero 
of  the  circle  graduation  in 
some  known  direction  as  the  FIG.  13. 

meridian,  or  in  an  east  and 

west  line.  For  use  on  shipboard,  the  instrument  is  made  with  a 
circle  of  the  silvering  removed  from  the  mirror,  so  that  with  the  chang- 
ing direction  of  the  ship  the  zero  of  the  nephoscope  can  be  constantly 
referred  to  a  compass  needle  placed  beneath  it. 

To  observe  the  direction  of  motion  of  a  cloud,  the  eye  is  so  placed 
as  to  see  constantly  the  reflection  of  the  knob  at  the  intersection  of  the 
cross-wires.  The  cross-wires  are  moved  into  such  a  position  that  a 
selected  point  on  a  cloud  is  seen  to  move  along  one  of  the  wires.  The 


46  METEOROLOGY. 

direction  of  the  wire,  as  indicated  on  the  circle,  is  the  direction  of  motion 
of  the  cloud.  On  some  instruments  there  are  concentric  circles  on  the 
mirror;  noting  the  times  of  transit  over  these  circles,  the  angular 
velocity  of  the  cloud  can  be  derived.  If  the  height  of  the  cloud  is 
known,  its  linear  velocity  can  be  computed. 

From  the  observation  of  clouds  at  sea,  the  direction  of  motion  and 
the  rate  of  motion  of  the  ship  being  known,  the  linear  velocity  and  the 
heights  of  the  clouds  can  be  computed. 

Amount  of  Cloudiness.  —  The  amount  of  cloud  is  estimated  by  the 
eye,  according  to  the  fractional  part  of  the  sky  covered.  The  estimate 
is  made  to  tenths,  sometimes  only  to  quarters.  On  the  first  scale  10 
indicates  a  sky  wholly  overcast ;  on  the  second  scale  the  same  condition 
is  indicated  by  4. 

Sunshine  Recorder.  —  For  finding  the  number  of  hours  the  sun  has 
been  shining,  an  instrument  called  a  "  sunshine  recorder  "  is  used,  shown 


FIG.  14. 


in  Fig.  14.  This  is  the  photographic  sunshine  recorder.  A  half-cylin- 
drical closed  box  contains  a  sheet  of  sensitized  paper.  The  image  of 
the  sun  is  formed  through  a  pin-hole.  As  long  as  the  sun  shines,  a 
continuous  line  is  traced  on  the  paper.  By  one  turn  of  a  screw  each 


ME  TE  OR  OL  0  GICAL  INS  TR  UMENTS. 


47 


day,  the  pin-hole  is  pushed  along  about  one-fourth  of  an  inch.  One 
sheet  of  paper  holds  a  record  for  a  month.  Another  form  of  sunshine 
recorder  consists  of  a  glass  sphere  4  inches  in  diameter  set  in  a  metal 
cup.  A  strip  of  paper,  with  rulings  representing  hours,  is  bent  around 
the  sphere.  The  sphere,  acting  as  a  lens,  burns  or  scorches  the  paper, 
making  a  record  of  the  number  of  hours'  duration  of  sunshine. 

Atmometer  or  Evaporometer.  —  Depth  of  evaporation  from  a  surface 
of  water  is  measured  by  means  of  an  evaporometer  or  atmometer.  A 
common  form  consists  of  a  round  dish  a  foot  in  diameter  and  four 
inches  deep.  The  loss  by  evaporation  is  ascertained  by 
weighing,  or  by  means  of  a  graduated  scale  inside  the 
dish.  The  scale  is  inclined  so  as  to  give  a  magnified 
reading  of  the  fall  of  surface.  The  vessel  is  kept  in  the 
open  air.  Readings  of  the  scale  are  made  at  intervals 
of  a  day.  Wind  and  rain  interfere  with  the  observation. 

The  Piche  evaporometer  is  shown  in  Fig.  15.  It  con- 
sists of  a  glass  tube  about  9  inches  long  and  0.4  of  an 
inch  inside  diameter,  graduated  to  equal  volumes.  It 
is  filled  with  water  and  the  end  covered  with  a  paper 
disk  1.2  inch  in  diameter,  held  in  place  by  a  metal  plate 
attached  to  a  spring  on  a  slitted  brass  collar  which 
moves  easily  along  the  tube.  The  water  is  fed  down 
from  above  by  gravity  to  the  paper,  from  which  it  evapo- 
rates both  on  the  upper  and  under  side.  The  amount  of 
evaporation  in  a  given  time  is  found  by  taking  the  dif- 
ference in  the  readings  of  the  top  of  the  column  of  water 
in  the  tube.  A  paper  surface  gives  off  one-third  more 
water  than  an  equal  extent  of  water  surface  in  a  dish, 
in  the  same  interval  of  time. 

For  an  exposed  paper  surface  of  1 1  square  centimetres 
the  number  of   cubic  centimetres  of  water  evaporated, 
multiplied  by  0.068,  gives  the  depth   of   evaporation   in   centimetres ; 
multiplying  by  0.0269  gives  the  depth  in  inches. 

With  a  wind  velocity  of  5  miles  an  hour  evaporation  is  2.2  times  as 
great  as  in  a  calm  ;  with  10  miles,  3.8  ;  with  15  miles,  4.9  ;  with  20  miles, 
5.7 ;  with  25  miles,  6.1 ;  with  30  miles,  6.3  times  as  great  as  for  a  calm. 


FIG.  15. 


METEOROLOGY. 


Electrometer.  —  A  quadrant  electrometer,  shown  in  Fig.  16,  is  used 
for  measuring  the  difference  in  electric  potential  between  the  ground 
and  the  air  at  a  height.  It  consists  of  a  cylindrical  metal  case  about  6 
inches  in  diameter  and  8  inches  high  surmounted  by  a  glass  tube 

8  inches  long  and  I  inch  in 
diameter.  There  is  a  circular 
glass  window  above  the  cylin- 
der one  inch  and  a  half  in 
diameter.  A  "needle,"  or 
disk,  made  of  thin  aluminium 
for  lightness,  is  suspended 
through  the  tube  and  cylin- 
der by  means  of  a  light 
platinum  wire.  Above  the 
needle  there  is  a  convex 
mirror,  and  below  the  mirror, 
on  the  same  wire,  are  three 
or  four  cross-pieces  of  wire, 
or  a  disk,  which  in  hanging 
down  through  the  cylinder 
dip  into  a  dish  of  sulphuric 
acid  at  the  bottom.  The 
resistance  of  the  liquid  to 
motion  of  the  cross-pieces 
immersed  in  it  diminishes 
the  oscillation  of  the  sus- 
pended needle  and  brings  it 
to  rest  when  set  in  motion. 
The  sulphuric  acid  serves  also 
FIG.  16.  to  keep  the  inside  of  the 

electrometer  dry  and  pre- 
serve the  insulation  of  the  glass  tube  and  the  glass  pillars  sustaining 
the  quadrants.  A  film  of  moisture  on  a  non-conducting  substance 
conducts  electricity  to  some  extent. 

The  quadrants  are  fastened  to  the  top  of  the  electrometer  inside 
by  glass  pillars.     The  quadrants  are  two  pieces  of  brass  joined  at  the 


METEOROLOGICAL  INSTRUMENTS.  49 

rounded  edges  with  an  interval  of  \  of  an  inch  between  them.  The 
quadrants  are  \  of  an  inch  apart ;  and  the  diagonally  opposite  pairs  are 
joined  by  a  wire. 

When  ready  for  use  the  needle  is  suspended  inside  the  quadrants  but 
not  touching  them.  The  electrometer  works  on  the  principle  of  repul- 
sion of  electricity.  One  pair  of  quadrants  is  joined  by  a  wire  to  one 
pole  of  a  battery  of  fifty  cells,  the  other  pair  to  the  other  pole. 
The  needle  is  connected  with  a  source  of  electricity  to  be  measured. 
When  the  needle  takes  on  a  greater  or  less  charge  of  electricity  it  turns 
through  a  greater- or  less  angle.  The  connection  of  the  needle  with 
a  source  of  electricity  is  by  means  of  a  wire  leading  from  the  place  and 
dipping  into  the  sulphuric  acid,  or  through  the  suspending  wire  by  the 
brass  cap  at  the  top  of  the  glass  tube.  The  resistance  to  the  rotation 
of  the  needle  is  in  the  torsion  of  the  wire. 

The  turning  of  the  needle  is  noted  by  the  reflection  of  a  spot  of 
light  from  the  convex  mirror.  A  lamp  is  so  arranged  that  a  beam  of 
light  thrown  on  the  mirror  is  reflected  back  to  a  scale  at  a  distance 
of  about  three  feet  from  the  electrometer.  As  the  mirror  turns,  the 
spot  of  light  moves  along  the  scale.  The  higher  the  potential  of  the 
source  of  electricity,  the  greater  the  deflection  of  the  needle,  until  it 
becomes  so  great  that  a  spark  flies  between  the  quadrants  and  the  needle. 

A  vessel  insulated  in  some  high  position  is  used  for  obtaining  the 
potential  of  the  air.  The  vessel  is  filled  with  water,  or  sand  in  cold 
weather,  which,  as  it  falls  from  the  vessel,  enables  it  to  take  on  the 
potential  of  the  air  where  contact  with  the  vessel  is  broken.  By  a 
connecting  wire  the  potential  is  transferred  from  the  vessel  to  the 
needle.  Where  the  wire  is  supported  at  any  points  between  the  vessel 
and  electrometer,  it  has  to  be  well  insulated.  Insulators  of  a  special 
form  are  used  for  this  purpose,  consisting  of  a  rod  of  glass  with  a  bell- 
shaped  glass  surrounding  the  rod  and  containing  sulphuric  acid  inside 
to  keep  the  glass  rod  free  of  any  coating  of  moisture.  The  water- 
dropper  is  also  insulated  by  a  special  form  of  glass  insulating  support. 
The  potential  of  the  air  is  determined  by  comparing  it  with  a  source  of 
some  known  potential,  such  as  a  large  number  of  cells  of  battery  joined 
in  series. 

River  Gauge.  —  A  river  gauge  is  used  for  ascertaining  the  height  of 


50  METEOROLOGY. 

river  surface  above  some  arbitrarily  selected  plane,  usually  at  or  some- 
where near  the  lowest  water  that  occurs  at  a  place.  The  reading  of 
the  water  surface  on  the  gauge  to  feet  and  tenths  is  called  the  stage  of 
water,  or  the  river  stage,  and  expresses  the  height  of  water  surface 
above  low  water.  When  possible,  without  too  great  expense,  a  river 
gauge  is  made  vertical.  A  bridge  pier  makes  the  most  desirable  loca- 
tion for  a  river  gauge.  The  gauge,  consisting  of  a  plank  2  inches 
thick  and  8  to  12  inches  wide,  is  fastened  to  the  pier,  and  made 
of  sufficient  length  to  cover  the  greatest  range  of  water  likely  to  occur. 
Sometimes  the  gauge  consists  of  a  portion  of  the  pier  dressed  down  to 
a  smooth  surface  so  as  to  receive  the  marking  and  numbering  of  the 
gauge.  River  gauges  are  marked  to  half-feet,  sometimes  to  tenths  of  a 
foot ;  only  the  foot  marks  are  numbered. 

When  a  river  gauge  cannot  be  set  vertically  on  a  bridge  pier,  it  is 
laid  according  to  the  slope  of  the  river  bank.  It  is  then  made  of  heavy 
timbers  with  a  strip  of  iron  nailed  along  the  top,  on  which  the  marks 
are  cut,  showing  vertical  heights  above  the  zero  of  the  gauge.  Some- 
times a  very  substantial  inclined  gauge  is  made  of  lengths  of  stone  with 
bars  of  railroad  iron  inlaid. 

Bench  Mark.  —  For  the  purpose  of  ascertaining  from  time  to  time 
any  changes  that  may  occur  in  the  level  of  the  zero  of  a  river  gauge  or 
any  of  its  marks,  a  bench  mark  is  established.  A  bench  mark  consists 
of  some  accessible,  presumably  permanent,  point  or  surface,  in  the 
vicinity  of  the  river  gauge,  the  difference  in  level  between  which  and 
the  zero  or  some  other  mark  on  the  gauge  is  known  by  actual  levelling 
between  the  two  with  a  spirit  level.  A  bench  mark  is  essential  in  case 
a  river  gauge  is  to  be  repaired  or  renewed  in  order  to  set  the  new  zero 
at  exactly  the  same  level  as  it  was  before.  On  a  bridge  pier,  the  top 
surface  of  the  largest  stone  accessible  in  the  top  course  of  masonry  is 
often  used  as  a  bench  mark.  Sometimes  the  bench  mark  is  the  top 
surface  of  a  stone  buried  in  the  ground  specially  for  the  purpose  of 
establishing  a  permanent  surface.  Stone  buildings  are  good  places  for 
permanent  bench  marks.  A  copper  bolt  set  in  the  wall  of  some  public 
building,  a  custom  house,  post  office  or  city  hall,  or  the  accessible  sur- 
face of  some  large  stone  in  a  building,  is  a  common  device  for  a  bench 
mark  in  large  cities. 


METEOROLOGICAL  INSTRUMENTS.  51 

In  establishing  a  river  gauge  it  is  customary  to  place  the  zero  at  the 
level  of 'the  lowest  water  known.  But  a  gauge  zero  once  established 
and  a  long  record  of  gauge  readings  made,  it  is  never  advisable  to 
change  the  zero  without  some  very  good  reason.  It  is  not  changed 
even  if  there  does  occur  a  stage  of  water  lower  than  any  before  known. 
When  a  stage  of  water  below  the  zero  occurs,  it  is  read  as  a  minus 
stage. 

Current  Meter.  —  A  current  meter,  used  for  observing  the  velocity  of 
the  flowing  water  in  a  river,  consists  of  a  propeller  wheel  which  revolves 
faster  the  greater  the  velocity  of  the  current.  The  Haskell  current 


FIG.  17. 

meter  with  register  is  shown  in  Fig.  17  one-tenth  of  the  actual  size. 
By  an  arrangement  which  makes  an  electric  circuit  every  revolution, 
the  revolutions  of  the  wheel  are  recorded  at  a  distance  when  the  instru- 
ment is  lowered  in  a  stream.  In  observing  a  current,  the  number  of 
turns  made  in  five  minutes  is  noted.  By  means  of  a  coefficient  pre- 
viously determined  for  the  instrument,  from,  a  known  rate  of  motion 
in  quiet  water,  the  velocity  of  water  in  the  river  can  be  determined.  A 
ship's  log  can  be  used  for  measuring  surface  velocities  of  water. 


CHAPTER   III. 

TEMPERATURE  AND  PRESSURE. 

Air  Temperature.  —  The  air  receives  one-third  to  one-half  of  its  heat 
directly  by  absorption  from  the  rays  of  the  sun  in  passing  through  it, 
and  the  rest  indirectly  by  conduction  from  the  earth's  surface  after  it 
is  heated  by  the  sun.  The  air  in  contact  with  the  earth  becomes 
heated.  On  account  of  its  diminished  density  the  heated  air  rises  and 
mingles  with  the  cooler  air  above.  This  process  of  convection  extends 
up  into  the  air  and  its  temperature  increases.  The  air  is  to  some 
extent  heated  by  radiation  from  the  earth. 

In  the  night  the  earth  and  the  air  cool  by  radiation.  The  air  radi- 
ates both  to  space  and  to  the  cooler  earth.  The  cooling  of  the  earth 
by  radiation  only  affects  the  air  for  a  slight  distance  above  it,  unless 
there  is  some  wind.  The  cooling  of  the  air  by  contact  with  the  earth 
produces  no  convection  as  in  the  case  of  heating,  so  there  is  no  cooling 
of  air  at  a  height  unless  there  is  a  mixture  and  contact  produced  by 
wind.  The  temperature  increases  upward  for  a  height  of  about  200 
feet.  Radiation  from  the  air  and  earth  is  favoured  by  a  small  amount 
of  moisture  in  the  air  and  by  a  clear  sky.  Cloud,  fog,  or  haze,  in  fact, 
almost  entirely  prevent  radiation.  As  the  air  diminishes  to  the  tem- 
perature of  its  dew-point,  the  formation  of  a  light  cloud  checks  farther 
radiation.  Radiation  from  the  earth  is  so  much  greater  than  from  the 
air  that  the  temperature  on  the  earth  may  be  eight,  ten,  or  even  twelve 
degrees  lower  than  the  air  a  few  feet  above  it.  When  the  temperature 
of  the  air  sinks  below  the  temperature  of  saturation  for  the  vapour  con- 
tained, fog  is  formed,  or  dew  is  deposited.  The  moisture  is  taken  out 
of  the  air  by  contact  with  the  colder  earth.  When  there  is  much  wind 
the  temperature  of  the  earth  does  not  get  much  lower  than  that  of  the 
air,  as  the  cold  of  the  earth  is  distributed  throughout  a  greater  depth  of 

52 


TEMPERATURE  AND  PRESSURE.  53 

air,  and  both  the  air  and  the  earth  are  at  nearly  the  same  temperature. 
In  this  case  no  dew  is  formed.  A  light  wind  is,  however,  favourable 
for  the  formation  of  copious  dew  by  renewing  the  supply  of  moisture- 
laden  air  in  contact  with  the  earth. 

Hoar  Frost. — When  the  temperature  of  the  dew-point  is  below  32°, 
instead  of  dew,  frost  is  formed.  It  is  not  frozen  dew.  Hoar-frost  is 
a  name  given  to  the  curious,  regular  figures  resembling  ferns  that  form 
on  objects,  especially  on  the  window-panes  in  houses. 

Rime.  —  Rime  is  a  thick,  heavy  frost  forming  on  objects  from  frozen 
rain  or  mist,  making  them  resemble  stalactites.  At  Niagara  Falls  tele- 
graph wires  near  the  cataract  sometimes  become  coated  in  this  way  to 
a  thickness  of  six  inches. 

Convective  action  diminishes  with  height  in  the  air.  Just  before 
sunrise  the  temperature  of  the  air  is  the  lowest  of  the  day.  It  receives 
from  the  sun  during  the  day  an  amount  of  heat  increasing  hour  by  hour 
until  noon.  The  radiating  power  of  the  air  increases  with  its  tempera- 
ture, and  the  mingling  process  by  which  it  is  cooled  also ;  but  not  to 
such  an  extent  as  the  heating  effect  of  the  sun.  The  consequence  is 
that  the  air  gradually  warms  up.  In  the  afternoon  the  heat  received 
decreases  in  amount,  but  continues  greater  than  the  cooling  effect  of 
radiation  and  convection  combined  until  about  two  o'clock,  when  the 
heating  and  cooling  effects  are  about  equal.  The  highest  temper- 
ature of  the  day,  therefore,  occurs  usually  at  two  o'clock  in  the  after- 
noon at  the  surface  of  the  earth.  The  time  varies  slightly  with  the 
season.  On  a  mountain  top  the  highest  temperature  is  just  about 
noon. 

Unsteadiness  of  Air.  —  For  an  hour  or  more  before  sunset  and  until 
some  time  after,  there  is  a  period  every  day  when  the  heat  received  by 
the  earth  from  the  sun  is  equal  to  the  amount  radiated  to  space.  There 
is  a  similar  period  in  the  morning,  from  before  until  after  sunrise, 
shorter,  however,  than  the  evening  period.  These  periods  are  marked 
by  steadiness  of  the  images  of  distant  objects  viewed  through  the  air. 
At  all  other  times  of  the  day  and  night,  except  sometimes  when  the 
sky  is  heavily  clouded,  in  viewing  objects  only  a  few  hundred  feet 
away,  there  is  a  tremulousness  about  the  images,  and  a  waviness  of 
outline,  similar  to  the  distortion  seen  in  looking  at  objects  across  the 


54  METEOROLOGY. 

surface  of  a  hot  stove.  Distant  objects  look  like  a  flag  in  a  gale  of 
wind.  This  is  owing  to  unequal  refraction  of  light  in  coming  through 
the  air  from  different  parts  of  an  object,  due  to  differences  of  density 
in  the  hot  and  cold  ascending  and  descending  currents. 

Unsteadiness  of  images  is  very  plainly  observed  in  sighting  over  long 
distances,  fifty  to  one  hundred  miles,  in  measuring  angles  in  the  geo- 
detic operation  of  triangulation.  It  is  only  at  about  sunrise  and  sunset, 
when  the  images  of  objects  are  steady,  that  angles  are  measured  where 
the  highest  accuracy  is  required.  The  period  of  steadiness  is  longer  in 
the  evening  than  the  morning.  The  unsteadiness  of  images  is  much 
greater  during  the  day  than  the  night.  On  days  when  the  sky  is  over- 
cast with  dense  clouds,  the  images  of  objects  are  steady  the  whole  day. 
The  air  is  always  unsteady  when  the  ground  is  covered  with  snow. 
The  quality  of  the  air  called  by  astronomers  good  "seeing,"  that  is,  the 
steadiness  of  star  images  as  seen  through  telescopes,  is  dependent 
on  varying  refractions  of  rays  of  light  coming  through  air  strata  of 
different  densities.  On  the  average,  the  "seeing"  grows  worse,  or 
the  unsteadiness  of  images  increases,  from  the  early  evening  to  mid- 
night. On  a  night  when  there  is  a  high  wind  the  "seeing"  is  usually 
bad. 

Daily  Range  of  Temperature. — The  difference  between  the  highest 
and  lowest  temperatures  of  the  day  is  called  the  "daily  range." 

It  varies  greatly  at  different  places  and  at  the  same  place  at  different 
times  of  the  year.  It  is  greatest  over  dry  arid  regions,  and  least  near 
coasts.  It  is  generally  greater  in  summer  than  winter.  San  Diego, 
Cal.,  is  an  exception.  While  the  average  range  is  9.4  degrees  in  June, 
it  is  13.6  in  January.  The  range  diminishes  in  going  from  the  equator 
towards  the  poles.  It  is  greater  on  mountain  summits,  up  to  a  certain 
height,  than  at  sea  level.  It  diminishes,  however,  with  height  in  the 
air  where  no  ground  is  interposed.  The  daily  range,  it  is  estimated, 
vanishes  at  a  height  of  about  29,000  feet :  it  is  less  on  a  cloudy  than  a 
clear  day.  A  partial  cloudiness  of  the  sky  makes  very  little  difference. 
A  convex  land  surface,  such  as  a  hill,  diminishes  the  range ;  a  concavity, 
such  as  a  valley,  increases  it.  The  greatest  range  occurs  at  places  having 
the  least  cloudiness. 

In  the  United  States,  east  of  the  Mississippi  River,  the  daily  range 


TEMPERATURE  AND  PRESSURE.  55 

of  temperature  varies  from  12  to  20  degrees,  being  about  15  on  the 
average ;  west  of  the  Mississippi  to  the  Rocky  Mountains,  it  varies  from 
20  to  35  degrees. 

The  average  at  New  York  is  11.5  degrees  in  June,  and  6.8  in  January. 
At  San  Francisco,  Cal.,  in  December  it  is  6.0,  and  at  Key  West,  Fla., 
6.6.  Over  the  plateau  region  of  the  Rocky  Mountains  the  range  is  very 
large.  At  Fort  Apache  (height,  5050  feet)  it  is  42.0  degrees.  At 
Campo,  Cal.  (height,  2710  feet)  it  is  45.4  from  June  until  November, 
and  occasionally  as  great  as  50.6.  On  Mount  Washington,  N.H. 
(height,  6279  feet),  the  range  is  18  degrees  in  January,  and  10.6  in  July. 
The  mean  for  the  year  is  about  the  same  as  at  sea  level.  The  tempera- 
ture from  its  lowest  about  sunrise,  increases  almost  steadily  to  the 
warmest  part  of  the  day  at  about  2  P.M.,  and  diminishes  steadily  until 
near  midnight.  It  then  remains  nearly  the  same,  diminishing  but  slightly 
to  the  minimum  near  sunrise.  In  a  very  dry  climate  the  diminution  is 
more  nearly  uniform  than  when  the  air  contains  a  good  deal  of  moisture. 
The  evolution  of  heat  in  forming  dew  retards  the  falling.  On  Pike's 
Peak,  Col.  (height,  14,134  feet),  the  range  is  14.3  degrees  in  July,  and 
1 1. 6  in  December.  The  mean  for  the  year  is  about  what  it  is  at  the 
base  of  the  mountain  at  Colorado  Springs  or  at  Denver.  The  highest 
temperature  on  a  mountain  corresponds  to  the  time  of  least  wind 
velocity,  and  the  lowest  to  the  time  of  greatest  wind. 

Temperature  Range  at  Sea.  —  Over  the  sea,  at  a  distance  from  shore, 
the  average  daily  range  in  the  temperature  of  the  air  is  only  3.2  degrees. 
The  daily  range  in  the  temperature  of  the  surface  water  of  the  ocean  is 
only  0.3  of  a  degree. 

Mean  Daily  Temperature. — The  mean  of  the  temperatures  at  every 
hour  of  the  day  at  a  place  is  taken  as  the  mean  daily  temperature. 
Hourly  observations  are  made  at  only  a  very  few  places  over  the  world. 
Approximate  values  of  the  mean  daily  temperatures  are  obtained  by 
various  short  processes.  The  mean  of  the  highest  and  lowest  tempera- 
tures of  the  day,  as  determined  with  the  maximum  and  minimum  ther- 
mometers, gives  a  mean  only  two  or  three  tenths  of  a  degree  too  high, 
on  the  average,  throughout  the  year  for  places  in  the  United  States. 
At  places  on  the  coast,  the  correction  to  the  mean  is  half  a  degree 
greater  than  for  places  inland.  At  Greenwich,  England,  the  same 


56  METEOROLOGY. 

mean  gives  the  temperature  0.6  of  a  degree  too  high,  being  1.2  too  high 
in  August,  and  0.3  too  high  in  January. 

One-half  the  sums  of  the  temperatures  at  nine  o'clock  in  the  morning 
and  nine  in  the  evening,  local  times,  gives  the  mean  for  the  day  too 
small  by  about  0.5  of  a  degree. 

One-half  of  the  eight  o'clock  morning  and  evening  local  time,  in  the 
United  States,  gives  a  temperature  a  few  tenths  of  a  degree  below  the 
mean  of  the  day,  and  in  central  Europe  gives  a  result  just  about  right. 

A  good  combination  which  gives  a  close  approximation  to  the  true 
value  of  the  daily  mean  for  most  places  is  one-fourth  of  the  7  A.M., 
2  P.M.,  and  twice  the  9  P.M.,  temperatures. 

The  mean  of  the  maximum  and  minimum  is  in  use  by  the  United 
States  Weather  Bureau  as  the  mean  of  the  day. 

The  daily  range  of  temperature  is  very  great  at  some  places  in  the 
tropics.  In  some  parts  of  India  the  range  is  so  great  and  the  tempera- 
ture goes  so  low  in  the  night  that  water  in  shallow  pans  is  frozen. 

To  the  west  of  Lake  Nyanza  in  Africa  the  rocks  become  so  greatly 
heated  during  the  day,  and  cool  so  suddenly  at  night,  that  cracks  with 
loud  reports  are  frequent.  (Livingstone.) 

Mean  Monthly  Temperature. — The  average  of  all  the  mean  daily 
temperatures  for  a  month  is  the  mean  monthly  temperature.  It  varies 
from  81°  near  the  equator  in  January  to  —26°  near  the  pole.  In  July  it 
varies  from  83°  in  latitude  10°  north,  to  34°  near  the  pole.  The  lowest 
monthly  temperature  in  the  tropics  is  about  66°. 

In  the  United  States  the  monthly  mean  varies  in  January  from  60°  in 
Florida  to  —  5°  in  North  Dakota  ;  in  July  from  82°  to  68°.  In  the  Ohio 
valley  the  mean  temperature  in  January  is  about  30°,  and  in  July  77°. 
At  San  Francisco  the  coldest  month,  January,  is  50°.  2,  and  the  warmest, 
September,  59°. 7.  At  Yuma,  Ariz.,  the  January  mean  is  50°,  and  the 
July  92°. 

Mean  Annual  Temperature.  —  The  mean  of  the  twelve  monthly  tem- 
peratures gives  the  mean  annual  temperature.  The  highest  mean 
annual  temperature  observed  at  any  place  in  the  world  is  at  Massowah, 
on  the  Red  Sea,  89°.2,  in  latitude  18°  north,  and  the  lowest  at  Fort 
Conger,  latitude  83°  north,  -5°. 

The    distribution    of    temperature    over   the   earth   is   not    strictly 


TEMPERATURE  AND  PRESSURE.  57 

dependent  on  latitude.  It  is  largely  modified  by  local  conditions,  and 
especially  by  the  prevalent  winds  at  a  place. 

In  Italy  the  winter  cold  is  less  intense  at  the  base  of  the  Alps  than 
on  the  open  plains  farther  south,  due  probably  to  protection  afforded  by 
the  mountains  during  north  winds. 

The  mean  annual  temperature  of  the  air  near  the  ground  for  the 
whole  surface  of  the  earth  is  about  58°.  Over  forest  land  the  mean 
temperature  increases  to  the  tops  of  the  trees. 

Ocean  Temperature.  —  The  surface  temperature  of  the  Atlantic  Ocean 
varies  from  84°  on  the  equator  to  28°  in  the  Arctic  Ocean.  The  freez- 
ing-point of  ocean  salt  water  is  at  27°. 5.  At  great  depths  in  the  ocean 
the  water  varies  in  temperature  from  32°.o  in  some  places  to  3 5°.  5  in 
others,  and  is  28°  at  all  depths  in  the  polar  seas.  The  temperature  of 
maximum  density  of  salt  water,  unlike  that  of  fresh  water,  is  greatest  at 
its  freezing-point.  The  average  temperature  of  a  column  of  ocean 
water  at  the  equator  from  the  surface  to  bottom  is  about  40°. 

On  the  United  States  coast  from  Cape  Hatteras  to  Boston,  the  tem- 
perature diminishes  in  winter  from  70°  to  30°,  the  greatest  change  to  be 
found  in  the  same  distance  at  any  place  in  the  world.  This  is  due  to  the 
warm  water  of  the  Gulf  Stream  that  touches  the  coast  of  North  Caro- 
lina, and  the  cold  current  from  the  north  following  down  the  New 
England  coast.  Shallow  places  distant  from  shore  have  cooler  water 
than  at  the  surface  of  deep  waters  adjacent.  This  is  very  marked  over 
the  Banks  of  Newfoundland,  in  contrast  with  the  Gulf  Stream  near  their 
eastern  edge.  In  a  distance  of  300  miles  there  is  a  change  in  tempera- 
ture of  33  degrees  in  the  surface  water.  The  cooler  water  over  the 
Banks  is  due  to  the  cold  water  in  the  currents  at  the  bottom  being 
forced  in  a  measure  to  the  surface  by  the  obstruction  formed  by 
the  Banks.  Near  shore  shallow  water  is  usually  warmer  than  deep 
water. 

From  latitude  40°  to  50°  the  ocean  freezes  near  the  shore.  Ice  forms 
all  over  the  polar  seas.  Ice  never  forms  at  any  place  to  a  greater  thick- 
ness than  six  feet,  except  by  breaking  and  piling  up. 

The  temperature  of  the  whole  body  of  water  in  oceans  and  lakes  is 
mainly  the  result  of  surface  temperature.  No  heat  is  received  from  the 
interior  of  the  earth,  even  at  the  greatest  depths. 


58  METEOROLOGY. 

Lake  Temperature.  —  In  fresh-water  lakes  in  middle  latitudes  the 
water  at  a  considerable  depth  is  always  at  a  temperature  of  39°, 
the  temperature  of  greatest  density  of  water.  As  the  water  cools  at  the 
surface,  it  becomes  heavier  than  the  warmer  water  below  and  descends, 
to  be  replaced  by  the  warmer  water.  When  it  cools  to  39°  it  no  longer 
descends.  Below  that  temperature  its  density  diminishes,  as  well  as 
above  it.  When  the  cold  of  winter  lasts  long  enough  to  cool  the  whole 
body  of  water  to  39°,  then,  when  the  surface  water  falls  lower,  freezing 
begins,  and  no  farther  cooling  goes  on  at  a  depth  no  matter  how  cold 
the  air  becomes,  except  the  slight  amount  of  heat  lost  through  the  ice 
by  conduction.  In  the  spring,  when  the  ice  melts,  and  the  surface 
water  warms,  there  is  no  downward  convection  of  heat.  The  heating  at 
a  depth  is  limited  to  the  heat  that  penetrates  to  a  depth  directly  from 
the  sun's  rays.  This  heating  extends  to  a  considerable  depth. 

River  Temperature.  —  In  rivers  the  constant  agitation  of  the  water 
in  flowing  prevents  freezing  until  the  whole  mass  of  water  cools  to  32° 
or  lower.  Ice  sometimes  forms  at  the  bottom  of  streams  on  stones,  and 
is  known  as  anchor  ice.  It  is  especially  noticed  in  forming  on  water- 
works inlet  pipes  projecting  into  streams,  which  furnish  water  supplies 
to  cities.  The  cause  of  the  formation  of  anchor  ice  is  not  understood. 
In  some  Siberian  rivers,  where  the  temperature  of  the  air  is  —60°,  ice 
forming  on  the  bottom  rises  and  thickens  the  surface  ice.  This  is  shown 
by  the  gravel  it  contains. 

Earth  Temperatures.  —  The  ground  at  a  few  feet  below  the  surface 
has  nearly  the  mean  annual  temperature  of  the  air  above  it.  This 
applies  to  places  where  rain  is  equally  distributed  throughout  the  year, 
and  the  earth  is  covered  with  snow  for  only  a  short  time.  Where  there 
is  a  wet  or  dry  season,  or  there  is  much  snow,  the  ground  may  be  above 
or  below  the  mean  temperature. 

The  range  of  temperature  in  the  earth  diminishes  rapidly  with  depth. 
The  daily  change  is  inappreciable  at  a  depth  of  3  feet,  in  middle  lati- 
tudes ;  in  the  tropics  at  a  depth  of  i  foot.  At  Brussels,  Belgium,  the 
annual  range  at  a  depth  of  3  feet  is  19  degrees ;  at  a  depth  of  13  feet, 
8  degrees ;  at  a  depth  of  26  feet,  2  degrees.  At  a  depth  of  26  feet  the 
highest  temperature  occurs  from  November  to  January,  the  lowest  in 
June  and  July.  The  annual  range  at  a  depth  of  60  feet  in  latitude  50°  is 


TEMPERATURE  AND  PRESSURE.  59 

inappreciable.  In  the  cellar  of  the  Paris  observatory,  at  a  depth  of  90.6 
feet,  the  temperature  is  invariably  5 3°. 3.  In  the  Mammoth  Cave,  Ken- 
tucky, the  temperature  is  uniformly  54°. 

Below  the  layer  of  invariable  temperature,  the  temperature  of  the 
earth  increases  downward  at  the  rate  of  one  degree  in  about  52  feet. 
There  is  not  an  increase  in  the  rate  of  change  in  going  down.  This 
shows  the  source  of  heat  is  not  near  the  earth's  surface. 

Where  the  mean  temperature  of  the  air  is  below  32°,  the  ground  at  a 
depth  is  frozen  all  the  year  round.  At  Jakutsk,  Siberia,  this  depth  is 
below  382  feet. 

Maximum  Temperatures.  —  The  highest  temperature  that  occurs  dur- 
ing the  day  is  called  the  "maximum  temperature." 

A  temperature  as  high  as  100°  occurs  occasionally  in  nearly  every 
part  of  the  United  States.  These  high  temperatures  never  occur  over 
more  than  two  or  three  hundred  thousand  square  miles  of  country  at 
the  same  time.  In  the  Ohio  valley  the  temperature  in  summer,  in  the 
hottest  part  of  the  day,  sometimes  reaches  105°.  At  Yuma,  Ariz.,  a 
temperature  of  118°  has  occurred.  In  Death  Valley,  Cal.,  the  highest 
summer  temperatures  are  about  122°.  Occasionally  the  lowest  occur- 
ring during  a  night  is  as  high  as  99°.  At  Pachpadra,  in  India,  a  tem- 
perature of  1 23°.  i  occurred  in  May,  1886.  At  Sialkot  I25°.o  occurred  in 
1873.  The  maximum  temperature  diminishes  with  height  in  the  air. 
On  Mount  Washington  the  highest  recorded  is  74° ;  on  Pike's  Peak  the 
highest  is  64°. 

Minimum  Temperatures.  —  The  lowest  temperatures  that  occur  vary 
widely  in  different  parts  of  the  country.  In  winter  the  temperature,  on 
rare  occasions,  goes  as  low  as  20°  in  northern  Florida  ;  10°  in  North 
Carolina,  Georgia,  Alabama,  Mississippi,  Louisiana,  and  eastern  Texas ; 
o°  in  Virginia,  Tennessee,  southern  Arkansas,  and  northern  Texas ; 
— 10°  in  Massachusetts,  Connecticut,  eastern  Pennsylvania,  West  Vir- 
ginia, Kentucky,  southern  Illinois,  northern  Arkansas,  and  Indian  Terri- 
tory; —  20°  in  Maine,  New  Hampshire,  Vermont,  northern  New  York, 
western  Pennsylvania,  Ohio,  Indiana,  northern  Illinois,  Missouri,  and 
Kansas;  —30°  in  northern  Michigan,  Wisconsin,  Iowa,  Nebraska,  and 
Colorado;  —40°  in  Minnesota,  South  Dakota,  and  Wyoming ;  —50°  in 
North  Dakota  and  Montana,  and  —60°  in  northern  Montana. 


6o 


METEOROLOGY. 


These  very  low  temperatures,  when  they  do  occur,  last  but  a  short 
time,  usually  not  more  than  a  fraction  of  an  hour.  In  the  eastern  and 
southern  parts  of  the  country,  they  do  not  occur  oftener  than  once  in 
1 5  years ;  in  the  north-west,  about  as  often  as  once  in  5  years. 

At  Fort  Conger,  in  the  Arctic  regions,  the  lowest  temperature  ob- 
served in  two  years  was  —66°.  2. 

In  Siberia  still  lower  temperatures  occur.  At  Jeniseisk  —73°.  5  has 
been  recorded. 

In  general,  lower  temperatures  occur  high  up  in  the  air  than  at  the 
surface  of  the  earth.  On  Mount  Washington  the  lowest  observed  is  —  50° ; 
on  Pike's  Peak,  —39°.  The  lowest  ever  observed  in  any  balloon  ascent, 
even  to  the  greatest  height,  five  miles  above  the  earth,  is  only  —40°. 

Freezing  Days.  — The  average  number  of  days  with  the  temperature, 
at  least  part  of  the  day,  as  low  as  32°  in  various  places  in  the  United 
States  are  as  follows  :  — 

TEMPERATURES  BELOW  32°. 


PLACES. 

NUMBER  OF 
FREEZING  DAYS 
IN  JANUARY. 

NUMBER  OF 
FREEZING  DAYS 
m  YEAR. 

2 

2 

Washington  City  

24. 

QO 

Boston,  Mass  

28 

y^ 

117 

2 

A 

St  Louis.  Mo  

24. 

82 

St  Vincent  Minn  

•21 

2O8 

El  Paso  Tex  

•j 

18 

^8 

Days  when  the  temperature  does  not  go  above  32°  are  called  "ice  days." 


Temperature  Variability. — The  average  difference  of  mean  tempera- 
ture at  a  place  on  successive  days,  regardless  of  whether  it  is  a  rise  or 
a  fall,  is  called  the  temperature  variability,  monthly  or  annual,  as  the 
case  may  be.  It  increases  from  south  to  north.  In  the  United  States 
the  annual  variability  is  6  degrees  in  Georgia,  7  in  New  England,  8  in 
the  Ohio  valley,  and  10  in  Dakota  and  Montana. 


TEMPERATURE  AND  PRESSURE.  6 1 

Dew.  —  Dew  forms  in  the  night,  and  even  in  the  day  when  the  air  is 
very  damp.  It  is  deposited  when  objects  cool  by  radiation  below  the 
temperature  of  saturation  of  the  air  in  contact  with  them.  A  strong 
wind  may  keep  the  air  along  the  ground  so  constantly  renewed  that  it 
may  not  fall  in  temperature  as  low  as  the  dew-point,  in  which  case  no 
dew  is  formed.  A  slight  agitation  of  the  air,  however,  conduces  to 
formation  of  copious  dew  by  renewing  the  supply  of  air  containing  the 
moisture.  Dew  is  not  formed  on  the  water  surface  of  lakes  or  ponds 
unless  below  the  temperature  of  39° ;  for,  when  the  water  cools,  it  sinks, 
and  other  warmer  water  takes  its  place.  The  depth  of  dew  that  forms 
in  a  year  in  the  eastern  part  of  the  United  States  is  estimated  to  be 
one-quarter  of  an  inch,  which  is  probably  too  low.  In  moist  climates, 
where  the  temperature  fall  is  great,  the  quantity  of  dew  is  also  great. 
On  the  Guinea  coast  of  Africa,  the  dew  at  times  runs  off  the  roofs  of 
huts  like  light  rain,  and  in  the  morning  mosquito  netting  is  wrung  out 
like  a  wet  towel.  In  the  Lake  Superior  region  the  dews  are  very  heavy, 
at  times  dripping  from  roofs. 

Frost.  —  When  the  temperature  is  below  32°,  frost  is  formed  instead 
of  dew.  A  slight  amount  of  cloudiness  or  haze  diminishes  radiation, 
and  diminishes  the  amount  of  dew  or  frost,  or  entirely  prevents  its 
formation.  An  artificial  haze  or  smudge  of  burning  straw  checks  radia- 
tion, and  is  sometimes  resorted  to  in  order  to  save  valuable  crops, — 
tobacco,  sugar-cane,  and  cranberries. 

The  damage  to  plants  by  frost  is  very  variable  at  different  times. 
Oftentimes  no  damage  is  done  by  frost  when  the  rise  to  freezing-point 
and  above  is  very  gradual. 

Frosts  are  called  " light"  when  the  temperature  sinks  no  more  than 
4  degrees  below  freezing-point.  When  the  temperature  falls  more 
than  four  degrees  below  freezing,  the  frost  is  called  " heavy,"  a  "killing 
frost,"  or  a  "black  frost." 

Frosts  may  occur  with  the  temperature  of  the  air  a  few  feet  above 
the  ground  12  or  16  degrees  higher  than  freezing-point. 

The  number  of  degrees  the  temperature  falls  below  32°  is  sometimes 
called  "degrees  of  frost." 

Frost  is  more  apt  to  occur  in  a  valley  than  on  a  hill  top,  as  the  wind 
is  less  apt  to  stir  up  the  air  in  a  valley. 


62  METEOROLOGY. 

The  higher  the  temperature  of  the  air  in  the  evening  and  the  greater 
the  amount  of  moisture  contained,  the  less  the  chance  of  frost  occurring 
during  the  night.  The  condensation  of  moisture  and  its  freezing  to  ice 
is  a  warming  process,  on  account  of  the  latent  heat  set  free,  and  tends 
to  retard  and  diminish  the  fall  of  temperature. 

The  temperature  of  the  dew-point  during  the  night  will  in  most  cases 
go  at  least  3  degrees  lower  than  the  dew-point  on  the  afternoon 
preceding. 

The  temperature  of  dew-point,  in  the  average  of  cases  where  the  air  is 
40°,  is  about  6  degrees  below  the  temperature  indicated  by  the  wet 
bulb  thermometer. 

First  frosts  in  the  autumn  are  subject  to  wide  variations  in  the  time 
of  occurrence  in  different  years.  On  the  average,  they  occur  as  follows 
in  the  United  States  :  — 

September    i.  —  North  Dakota,  Minnesota,  Wisconsin,  northern  Michigan. 

September  15. —  Nebraska,  northern  Illinois,  southern  Michigan,  northern  New 
York. 

October  i .  —  Kansas,  northern  Missouri,  central  Illinois,  Indiana,  Ohio,  Penn- 
sylvania, Connecticut,  Rhode  Island,  Massachusetts. 

October  15.  —  Indian  Territory,  southern  Missouri,  Tennessee,  Kentucky, 
Virginia,  Maryland,  North  Carolina. 

November  i.  —  The  northern  part  of  Louisiana,  Mississippi,  Alabama,  Georgia, 
and  South  Carolina. 

November  15.  —  Central  Louisiana,  and  the  southern  parts  of  Mississippi, 
Alabama,  and  Georgia. 

December    i.  —  Coast  of  Gulf  of  Mexico,  and  northern  Florida. 

December  15.  —  Central  Florida. 

The  average  last  frosts  in  the  spring  occur  as  follows  :  — 

February      i .  —  Central  Florida. 

February    15.  —  Gulf  Coast  and  northern  Florida. 

March          i .  —  South  Carolina,  southern  Georgia,  southern  Alabama,  southern 

Mississippi,  northern  Louisiana,  and  southern  Texas. 
March         15.  —  South  Carolina,  and  the  central   part  of  Georgia,  Alabama, 

Louisiana,  and  Texas. 
April  i .  —  North  Carolina,  Tennessee,  Arkansas,  Indian  Territory,  northern 

Texas. 


TEMPERATURE  AND  PRESSURE.  63 

April   15.  —  Massachusetts,  Maryland,  Virginia,  West  Virginia,  Kentucky,  In- 
diana, Illinois,  Missouri,  and  Kansas. 

May      i. : — Northern  New  York,  Michigan,  Wisconsin,  Iowa,  Nebraska. 
May     15.  —  North  and  South  Dakota,  Minnesota,  northern  Michigan. 
June       i .  —  North  of  Dakota. 

Frosts  after  April  ist  are  apt  to  aftect  the  wheat-growing  in  the 
winter-wheat  belt  disastrously.  This  region  comprises  Missouri,  Illinois, 
western  Kentucky,  north-western  Tennessee,  and  southern  Michigan. 
Corn,  which  is  hardier,  and  not  so  much  affected  by  frost,  is  grown 
principally  in  Kansas,  Iowa,  and  Nebraska. 

Frosts  in  the  country  to  the  north  of  Dakota  cause  the  failure  of 
about  one  wheat  crop  out  of  every  three. 

First  frosts  occur  in  some  years  as  much  as  twenty-six  days  before  or 
after  the  average  times  of  first  occurrence,  and  last  frosts  twenty-six 
days  before  or  after  the  average  times  of  last  occurrence.  The  whole 
range  of  fifty-two  days  in  the  times  of  first  occurrence,  and  the  same 
range  in  the  times  of  last  occurrence  of  frost,  takes  place  usually  within 
a  period  of  twenty  years. 

Lake  Climate.  —  Where  winter  temperatures  sink  considerably  below 
freezing-point,  lakes  produce  an  appreciable  effect  on  climate  in  the 
vicinity.  The  great  specific  heat  of  water,  and  the  fact  that  throughout 
the  whole  depth  of  a  lake  it  must  cool  to  39°  before  the  surface  freezes, 
and  the  fact  that  there  is  a  great  amount  of  heat  given  off  in  the  transi- 
tion to  ice,  keep  the  air  temperature  in  the  vicinity  higher  than  would 
otherwise  be  the  case.  In  spring,  the  ice  has  an  opposite  effect,  and 
keeps  the  air  in  its  vicinity  cool. 

The  difference  due  to  this  cause  in  the  time  of  leafing  of  trees  in 
spring  on  the  shore  of  Lake  Ontario  and  a  few  miles  inland  is  a  week 
or  more. 

To  melt  a  kilogramme  (2.2  pounds)  of  ice  requires  79.06  kilogramme- 
calories  of  heat.  On  the  English  system  of  units,  to  melt  a  pound  of 
ice  requires  142  units  of  heat. 

Temperature  and  Height.  —  The  temperature  diminishes  with  ascent 
in  the  air.  The  rate  of  diminution  from  the  equator  to  60°  north  lati- 
tude is,  on  the  average,  0.321  degrees  for  every  100  feet  of  ascent  for  the 
first  few  thousand  feet  from  the  surface  of  the  earth.  There  are  local 


64  ME  TE  OR  OL  O  G  Y. 

variations  from  0.289  to  0.420  of  a  degree  in  different  places.  The 
variation  has  no  relation  to  latitude  as  far  as  can  be  observed.  The 
difference  between  the  top  of  the  Eiffel  Tower,  1000  feet,  and  62  feet 
above  the  ground  is  1.6  degrees.  The  average  diminution  is  one-third 
greater  in  summer  than  winter.  Between  the  top  and  bottom  of  Ben 
Nevis  in  Scotland,  4368  feet,  the  difference  is  0.363  of  a  degree  for  100 
feet ;  the  greatest  is  0.405  in  April,  and  the  least  0.326  in  December. 
Between  the  top  and  bottom  of  Mount  Fugiamo  in  Japan,  a  difference  of 
12,087  feet>  tne  average  rate  of  diminution  is  0.305  of  a  degree  per  100 
feet.  Between  the  top  of  Pike's  Peak  and  Denver  it  is  0.347. 

Taking  the  atmosphere  as  a  whole,  up  to  the  limit  of  the  free  surface 
of  the  air,  the  rate  of  upward  diminution  of  temperature  must  diminish 
with  the  increase  of  latitude.  The  air  at  a  very  great  height  is  probably 
everywhere  at  the  same  very  low  temperature.  Consequently  the  rate 
of  diminution  upward  at  the  equator  must  be  greater  than  at  the  pole, 
as  the  surface  temperature  at  the  pole  is  low  as  compared  with  that  at 
the  equator. 

PRESSURE    OF    AIR. 

Air  Pressure.  —  Air  pressure  varies  widely  at  different  places,  at  the 
same  instant  of  time.  It  is  also  very  different  at  the  same  place  from 
time  to  time.  The  average  barometric  pressure  over  the  northern 
hemisphere,  reduced  to  sea  level,  is  29.95  inches. 

Diurnal  Oscillation.  —  The  pressure  is  subject  to  a  double  oscillation 
in  the  course  of  a  day.  It  is  the  highest  for  the  day  at  ten  o'clock  in 
the  morning,  and  diminishes  from  that  time  until  three  in  the  afternoon  ; 
it  then  increases  until  nine  o'clock  in  the  evening,  not,  however,  going 
as  high  as  in  the  morning ;  from  this  it  diminishes  until  three  o'clock  in 
the  morning,  but  does  not  go  as  low  as  in  the  afternoon  ;  then  it 
increases  again  until  ten  o'clock.  This  is  for  places  not  very  much 
above  sea  level.  The  continental  type  of  pressure  tends  to  but  a  single 
maximum  and  minimum  for  the  day.  The  diurnal  oscillation  in  a  very 
dry  climate  tends  to  a  single  swing. 

There  are  wide  departures  during  the  prevalence  of  storms.  There 
are  slight  variations  in  the  amount  of  change  for  various  regions,  char- 
acteristic of  oceanic  and  continental  climate.  On  high  mountains  the 


TEMPERATURE  AND  PRESSURE.  6$ 

variation  of  pressure  is  very  different  from  what  it  is  at  sea  level, 
being  complicated  with,  or  largely  dependent  on,  the  daily  range  of 
temperature. 

Variation  with  Latitude.  —  The  average  daily  range  of  pressure  dimin- 
ishes from  the  equator  to  the  poles.  In  the  tropics  the  daily  variation 
of  pressure  is  very  regular,  so  much  so  that  it  is  said  one  can  tell  the 
time  of  day,  within  twenty  minutes,  from  the  reading  of  a  good  barom- 
eter. The  least  deviation  from  the  regular  daily  march  of  pressure 
is  evidence  of  a  storm  in  the  vicinity. 

At  Calcutta,  India,  latitude  24°,  the  daily  range  of  pressure  is  0.116 
of  an  inch;  at  Greenwich,  England,  latitude  52°,  0.020;  at  Fort  Conger, 
latitude  83°,  only  o.oio  of  an  inch.  Over  the  Pacific  Ocean,  from  12° 
north  to  12°  south,  the  range  is  0.087  °f  an  inch. 

In  the  United  States  the  range,  as  determined  from  twelve  years'  ob- 
servations, varies  from  0.117  °f  an  incn  at  San  Antonio,  Tex.,  and 
0.068  at  Dodge  City,  Kan.,  to  0.038  at  Bismarck,  N.  Dak.  It  requires 
the  average  of  years  of  observation  to  disentangle  the  diurnal  range 
effect  from  the  irregular  oscillations  accompanying  storms. 

The  range  increases  from  winter  to  summer.  The  range  increases 
inland,  being  0.061  at  Philadelphia,  0.068  at  St.  Louis,  0.072  at  Denver, 
0.079  at  Salt  Lake  City,  and  0.094  at  Winnemucca,  Nev.,  and  0.058 
at  San  Francisco.  It  is  0.129  at  Yuma,  Ariz.  In  approaching  the 
great  lakes  the  range  diminishes  ;  while  at  Albany  the  range  is  0.074, 
at  Buffalo  it  is  only  0.047,  at  Chicago,  0.046.  The  principal  minimum, 
in  January,  occurs  along  the  Atlantic  coast  and  in  the  region  of  the 
great  lakes  at  about  2.30  P.M.  As  the  year  advances  it  becomes  later, 
occurring  at  5  P.M.  in  June  along  the  coast,  while  in  the  lake  regions  it 
is  delayed  to  5.45  and  6  P.M.  In  the  Pacific  coast  regions  the  winter 
minimum  occurs  at  4  P.M.,  and  the  summer  at  6  P.M.  At  inland  stations 
the  minimum  occurs  from  3  to  3.30  P.M.,  and  in  summer  two  hours 
later. 

Effect  of  Moon  on  Pressure.  —  The  tides  in  the  atmosphere  produced 
by  the  moon  have  an  effect  on  the  daily  range  of  pressure.  At  Batavia, 
Java,  latitude  6°  n',  the  means  of  fifteen  years'  observations,  arranged 
according  to  lunar  hours,  —  that  is,  by  the  hour  angle  of  the  moon  from 
meridian,  —  show  the  difference  between  the  greatest  and  least  pressure 


66  METEOROLOGY. 

due  to  the  moon  to  be  0.0x346  of  an  inch ;  at  Singapore,  latitude  i°  11', 
0.0064,  and  at  St.  Helena,  latitude  15°  37',  0.004.  At  the  two  latter 
places  the  maximum  is  precisely  at  the  time  of  the  moon's  culmination ; 
at  Batavia  50  minutes  later. 

Cause  of  Range  of  Pressure.  —  On  cloudy  days  the  range  of  pressure 
is  only  half  as  great  as  on  clear  days.  Over  the  land  it  is  slightly 
greater  than  over  the  ocean.  The  range  seems  to  be  composed  of  two 
distinct  oscillations  superposed,  one  depending  on  the  position  of  a 
place,  and  the  other  nearly  the  same  at  all  places  over  the  earth.  One 
component,  the  greater,  is  due  to  changes  in  the  lower  air  allied  to 
temperature ;  the  other  to  changes  at  a  high  altitude  in  the  air. 

The  cause  of  the  daily  range  of  pressure  is  not  fully  understood.  No 
theory  as  yet  proposed  accounts  for  all  the  facts.  There  seems  to  be 
no  doubt  it  is  dependent  somewhat  on  the  quantity  of  moisture  contained 
in  the  air,  and  the  fact  that  moist  air  takes  more  heat  from  the  direct 
rays  of  the  sun  than  dry  air.  Yet  at  Jacobabad,  India,  it  is  0.187,  in 
January  and  July,  where  the  air  is  always  dry.  At  Aden,  Arabia,  where 
it  is  dry  all  the  year,  it  is  0.084  m  January  and  0.163  in  August.  At 
Bombay  in  January,  which  is  dry,  it  is  0.119,  and  in  July,  which  is  wet, 
it  is  0.067.  The  daily  change  is  less  in  areas  of  high  pressure,  which  are 
dry,  than  in  areas  of  low  pressure.  If  it  were  due  to  the  heating  effect 
of  the  sun  on  the  surface  of  the  earth,  it  would  be  very  much  greater 
over  the  land  than  sea,  which  is  not  the  case.  In  deep  narrow  valleys 
there  is  a  great  increase  of  the  daily  range,  the  change  being  much 
larger  than  on  open  plains.  At  Gies  and  Klagenfurt  in  Austria,  though 
in  latitude  45°,  the  range  is  nearly  as  great  as  in  the  tropics.  In  Death 
Valley,  southern  California,  200  to  400  feet  below  sea  level,  the  range 
is  o.i  86  of  an  inch  in  summer.  This  is  due  to  the  excess  of  cooling 
effect  in  valleys  and  their  lower  temperature  in  the  night  time  as  com- 
pared with  plains. 

On  mountains  the  highest  pressure  occurs  at  noon.  The  daily  range 
is  almost  wholly  due  to  the  effect  of  the  heating  of  the  air  between  the 
summit  and  base  of  mountain.  The  air  between  the  base  and  summit 
expands  by  heating,  and  some  of  the  air  is  pushed  above  the  mountain, 
so  that  there  is  a  greater  quantity  of  air  above  the  barometer  at  a  warm 
than  at  a  cold  part  of  the  day.  On  Mount  Washington  in  January  the 


TEMPERATURE  AND  PRESSURE.  6/ 

pressure  is  0.50  of  an  inch  lower  than  in  July ;  on  Pike's  Peak  0.59  of  an 
inch  lower.  On  Ben  Nevis,  Scotland,  4368  feet,  a  difference  of  17.4 
degrees  increases  the  pressure  0.143  of  an  inch. 

Monthly  Range  of  Pressure.  —  The  difference  between  the  highest 
and  lowest  pressure  in  a  month  is  the  monthly  range.  It  increases 
with  latitude.  At  Key  West,  Fla.,  it  is  0.35  of  an  inch,  and  at  East- 
port,  Me.,  1.16.  At  Brownsville,  Tex.,  it  is  0.55,  and  on  the  same 
meridian  at  St.  Vincent,  Minn.,  1.02.  The  range  is  least  for  the  month 
of  July  and  greatest  for  January. 

Annual  Range  of  Pressure.  —  The  difference  between  the  greatest 
and  least  pressures  occurring  in  a  year  is  the  annual  range.  At  Key 
West,  in  20  years,  it  has  been  1.176  inches;  at  New  York,  2.201  ;  at 
Eastport,  2.523;  at  Brownsville,  1.896;  and  at  Chicago,  1.775.  At 
Toronto,  Canada,  the  range,  in  46  years,  has  been  2.77.  Pressures 
below  the  average  occur  at  the  times  storms  are  prevailing,  and  pres- 
sures above  the  average  after  they  are  past.  This  is  especially  the 
case  in  winter. 

As  a  rule,  in  the  United  States,  pressures  rarely  go  as  low  as  28.9 
inches,  even  in  the  greatest  storms,  and  after  they  are  past  it  rarely 
goes  higher  than  30.7  inches.  Cases  of  much  lower  and  higher  pressure 
do,  however,  occur  occasionally. 

In  a  typhoon  in  the  China  Sea  the  pressure  has  been  known  to  sink 
as  low  as  27.04  inches.  At  Reikiavik,  Greenland,  a  pressure  of  27.25 
inches  occurred  February  4,  1824.  February  5,  1870,  on  the  steamer 
Tarifa,  in  latitude  51°  north  and  longitude  24°  west,  a  pressure  of 
27.33  inches  was  observed. 

Pressures  sometimes  rise  to  3 1 .00  inches.  At  Fort  Assiniboine,  Mont., 
the  pressure  reached  31.21  inches  January  6,  1886.  At  Semipalatinsk, 
Siberia,  December  1 6,  1877,  the  pressure  reduced  to  sea  level  was  31.72 
inches  (height  above  sea  level,  597  feet).  At  Barnaul,  Siberia,  Decem- 
ber 17,  1877,  the  pressure  was  31.64  inches.  The  average  of  high 
pressures  in  Siberia  in  January  is  31.10  inches. 

Distribution  of  Pressure.  —  The  average  distribution  of  pressure  at 
sea  level  over  the  surface  of  the  earth  is  determined  by  the  general  cir- 
culation of  the  air.  At  a  height  in  the  air  the  pressure  is  dependent 
somewhat  on  the  temperature  of  air  in  the  lower  layers.  When  the 


68  METEOROLOGY. 

temperature  is  high,  there  is  a  greater  quantity  of  air  at  a  height  than 
when  it  is  cold.  There  is  a  constant  interchange  between  the  air  at  the 
equator  and  the  poles.  The  rotation  of  the  earth  diverts  a  current  to 
the  right  of  the  direction  in  which  it  is  moving,  no  matter  what  the 
direction.  The  deflective  effect  varies  with  the  velocity  of  current  and 
the  sine  of  the  latitude  of  the  place.  The  consequence  is,  there  is  a 
tendency  to  the  formation  of  low  pressure  on  the  left  side  of  a  current, 
in  opposition  to  the  general  tendency  of  air  to  move  from  a  place  where 
the  pressure  is  high  to  where  it  is  low  until  the  pressure  is  equalized. 
For  a  given  velocity  of  current  in  a  given  latitude  there  is  thus  a  certain 
increase  of  pressure  from  the  left  to  the  right  side  of  the  current.  The 
increase  is  greater  the  higher  the  latitude  and  the  greater  the  velocity 
of  the  current.  When  a  current  of  air  describes  a  circle  on  the  earth, 
or  a  mass  of  air  is  in  rotation,  there  is  therefore  a  lowering  of  pressure 
or  an  increase  of  pressure  produced  in  the  centre  of  the  mass  by  the 
effect  of  the  earth's  rotation,  depending  on  the  direction  of  rotation  of 
the  air. 

There  is  a  general  tendency  of  the  whole  mass  of  air  north  of  latitude 
40°  to  move  from  west  to  east  around  the  earth.  The  consequence  is, 
there  is  a  permanent  area  of  low  pressure  in  the  vicinity  of  the  pole. 
The  stronger  the  air  currents  the  greater  the  lowering  of  pressure,  as 
in  winter,  with  the  greatest  difference  of  temperature  between  the 
equator  and  the  pole.  There  is  also,  as  a  consequence  of  this,  the  heap- 
ing up  of  the  air  and  a  high  pressure  about  latitude  30°. 

Average  January  Pressure.  —  In  January  the  average  pressure  over  the 
north  Atlantic  Ocean  in  the  vicinity  of  Iceland  is  29.55  inches.  In  the 
southern  hemisphere,  in  its  winter,  there  is  a  very  extensive  area  of 
pressure  29.3  inches  or  less  surrounding  the  pole.  In  January,  in  Siberia, 
the  pressure  is  30.5  inches.  In  the  north  Pacific  Ocean,  off  the  coast  of 
Alaska  and  Kamtchatka,  the  pressure  in  January  is  29.55  inches,  and 
in  the  United  States  it  is  30.2  inches.  The  pressures  at  intervening 
places  shade  gradually  between  these  values. 

The  circulation  of  the  air  over  that  part  of  the  earth's  surface  where 
the  trade-winds  prevail,  and  the  anti-trades  above  them,  is  almost  inde- 
pendent of  the  circulation  north  of  latitude  40°.  The  trades  and  anti- 
trades form  a  circulatory  system,  there  being  a  generally  ascending 


TEMPERATURE  AND  PRESSURE.  69 

current  at  the  region  of  calms  near  the  equator,  and  a  generally  descend- 
ing current  at  about  latitude  35°.  The  deflecting  force  produced  by  the 
earth's  rotation  is  small  near  the  equator.  At  the  equator,  in  January, 
the  pressure  is  29.91  inches,  and  at  latitude  35°  north,  30.14,  at  sea 
level ;  but  on  ascending  in  the  air  to  10,000  feet,  the  pressures  become 
gradually  equal  from  the  equator  to  35°  north,  and  on  further  ascent 
they  are  found  to  diminish  in  the  opposite  direction.  On  the  mountain 
of  Antisana,  in  Ecuador,  near  the  equator,  the  pressure  at  a  height  of 
13,000  feet  is  18.55  inches,  while  at  the  same  height  on  Pike's  Peak,  Col, 
in  latitude  39°,  it  is  18.04  inches. 

The  area  of  relatively  high  pressure  of  varying  width  around  the  earth 
about  latitude  355is  somtimes  known  as  the  sub-tropical  zone  of  high 
pressure. 

Average  July  Pressure.  —  In  July  there  is  an  area  of  low  pressure 
29.4  inches  in  Asia,  from  Mooltan  to  Muscat,  a  region  east  of  the 
Caspian  Sea.  Around  the  north  pole  there  is  a  very  large  area  of  29.9- 
inch  pressure.  At  latitude  30°,  in  the  Atlantic  Ocean,  the  pressure  is 
30.2  inches,  and  in  the  Pacific,  30.3. 


CHAPTER   IV. 

EVAPORATION,  CLOUDS,  RAIN,  AND  SNOW. 

Evaporation.  —  The  air  always  contains  some  vapour  of  water,  trans- 
parent and  colourless  like  the  other  component  gases.  When  the  vapour 
condenses,  it  becomes  visible  as  fog,  cloud,  rain,  snow,  or  hail.  From 
any  water  surface  vapour  passes  into  the  air  by  evaporation.  The  rate 
of  water  evaporated  depends  on  the  temperature  of  the  water,  the  dry- 
ness  of  the  air,  and  the  velocity  of  the  wind.  With  a  high  barometric 
pressure  evaporation  is  somewhat  less  than  with  a  low  pressure.  Evapo- 
ration requires  heat.  The  number  of  units  of  heat  required  to  raise  the 
temperature  of  a  kilogramme  of  water  from  o°  centigrade  to  boiling- 
point,  and  convert  it  to  vapour,  is  607.  To  convert  it  from  boiling-point 
to  vapour  requires  537  heat-units.  On  the  English  system,  to  vaporize 
a  unit  weight  of  water  requires  967°  F.  This  heat  disappears  on  evapo- 
ration, and  is  rendered  latent.  It  reappears  again  on  the  condensation 
of  the  vapour.  A  quantity  of  vapour  cannot  be  made  liquid  unless  there 
is  some  means  of  disposing  of  its  latent  heat.  From  this  it  results  that 
condensation,  and  cloud  or  rain  formation,  are  usually  gradual  processes. 
When  the  radiation  of  heat,  or  the  mixture  of  warm  moist  air  with  cold 
air,  causes  the  formation  of  cloud,  fog,  or  haze,  the  reappearing  latent 
heat  of  condensation  retards  further  condensation  until  the  temperature 
can  be  lowered. 

Pure  vapour  is  condensed  by  subjecting  it  to  pressure.  In  the  free 
air,  however,  where  vapour  and  gas  are  mixed,  pressures  never  increase 
sufficiently  to  produce  appreciable  condensation.  Any  increase  of  pres- 
sure beyond  that  for  the  temperature  of  saturation  condenses  part  of 
the  vapour.  The  latent  heat  diminishes  with  the  temperature  of 
vaporization. 

70 


EVAPORATION,    CLOUDS,   RAIN,   AND  SNOW.  71 

Beyond  certain  very  high  temperatures,  no  pressure,  however  great, 
can  render  vapour  liquid.  This  temperature,  called  the  "critical 
temperature,"  is  different  for  different  liquids ;  for  water  it  is  684°  F. 

The  depth  of  water  that  will  evaporate  in  the  course  of  a  year  varies 
widely  in  different  places ;  at  Boston,  as  observed  with  a  pan  evaporom- 
jter  in  the  shade,  it  is  39  inches;  in  the  Ohio  valley,  40  inches;  over 
a  wide  extent  of  country  in  the  southern  parts  of  New  Mexico,  Arizona, 
and  California  it  is  about  100  inches;  at  Cumana,  South  America,  136 
inches. 

In  full  exposure  to  sun  and  wind  the  evaporation  will  be  still  larger. 
The  actual  water  evaporated  from  soil,  running  water,  and  lakes  is 
considerably  less. 

Daily  Change  of  Vapour  Pressure.  —  There  is  a  daily  variation  in  the 
pressure  of  vapour  in  the  air.  Near  the  sea-coast  the  variation  corre- 
sponds nearly  with  the  daily  change  of  temperature ;  inland,  it  is  some- 
what different.  The  upward  tendency  of  the  lower  air  when  heated 
during  the  daytime,  and  its  mixture  with  the  upper  air,  is  the  principal 
cause  of  the  daily  variation. 

In  the  daytime  the  vapour  is  carried  away  from  the  surface  of  the 
earth  and  disseminated  through  the  upper  air  so  fast  that  even  in  the 
vicinity  of  the  sea  the  vapour  pressure  diminishes  as  the  day  advances. 

At  places  inland  the  vapour  pressure  decreases  decidedly  during  the 
day,  as  there  is  but  little  evaporation  from  the  ground  to  keep  up  the 
supply  of  vapour.  The  vapour  pressure  diminishes  from  the  equator  to 
the  poles. 

In  the  tropics,  over  the  ocean,  the  vapour  pressure  varies,  on  the 
average,  from  0.639  °*  an  mcn  at  4  A-M->  to  0.679  at  2  P.M. 

At  Batavia,  on  the  island  of  Java,  6°  south  of  the  equator,  the  lowest 
vapour  pressure  at  sunrise  is  about  0.783  of  an  inch ;  it  increases  by 
nine  o'clock  to  0.823;  diminishes  to  0.815  by  eleven  o'clock,  then 
rises  to  0.838  of  an  inch  by  7  P.M.,  and  then  diminishes  regularly  until 
morning. 

Yearly  Change  of  Pressure.  —  The  yearly  change  in  vapour  pressure 
is  very  similar  to  the  yearly  change  in  temperature.  The  difference 
between  the  greatest  and  least  pressure  is  less  on  the  coast  than  inland, 
and  is  smaller  in  the  tropics  than  in  the  temperate  zone. 


72  METEOROLOGY. 

The  average  vapour  pressure  at  Jacksonville,  Fla.,  in  January  is  0.360 
of  an  inch,  and  in  July,  0.732;  at  Washington  City,  in  January,  0.124, 
and  in  July,  0.616;  at  Boston,  in  January,  0.088,  and  in  July,  0.499; 
at  New  Orleans,  in  January,  0.310,  and  in  July,  0.810;  at  St.  Louis,  in 
January,  o.ioi,  and  in  July,  0.707;  at  St.  Vincent,  Minn.,  0.020  in 
January,  and  0.499  in  July;  at  El  Paso,  Tex.,  0.141  in  January,  and 
0.448  in  July. 

Vapour  pressure  diminishes  rapidly  with  ascent  in  the  air.  The 
air,  up  to  a  height  of  one  mile,  contains  40  per  cent  of  all  the 
moisture  in  the  whole  atmosphere ;  two  miles,  69  per  cent ;  three 
miles,  8 1  per  cent ;  four  miles,  89  per  cent ;  and  five  miles,  94 
per  cent. 

The  small  amount  of  moisture  in  mountain  regions  lends  the  air  a 
peculiar  transparency.  At  Denver,  Col.,  mountain  peaks  forty  miles 
or  more  away  seem  only  a  few  miles  distant.  Such  dry  air  is  entirely 
lacking  in  colour-tone. 

CLOUDS. 

Cloud  Formation.  —  Cloud  forms  when  the  air  cools  below  its  temper- 
ature of  saturation  (for  the  vapour  it  contains),  either  by  dynamically 
cooling  in  ascent,  or  by  convective  intermixture  with  colder  air  in 
ascending  or  by  radiation  from  the  air.  Low-lying  clouds  near  the  surface 
of  the  earth  usually  result  from  the  air  cooling  below  the  dew-point  by 
radiation.  The  degree  of  cloudiness  depends  jointly  on  the  amount  of 
vapour  in  the  air  and  the  activity  of  convection,  which  depends  in  turn 
on  the  decrease  of  temperature  with  altitude.  When  the  air  up  to  a 
great  height  is  warm,  there  may  not  be  any  clouds,  even  with  a  good 
deal  of  vapour.  The  ascending  currents  of  air  around  mountains  in  the 
daytime  cause  the  formation  of  cloud-caps  around  their  summits.  The 
tops  of  high  mountains  are  seldom  free  from  clouds.  The  depth  of  a 
layer  of  cloud  is  not  usually  more  than  half  a  mile.  Cumulus  clouds, 
however,  are  sometimes  several  miles  in  thickness. 

Classification  and  Nomenclature.  —  Clouds  are  broadly  classified  into 
upper  and  lower,  according  to  height  in  air.  Cirrus  is  the  highest,  and 
stratus  the  lowest.  Ten  varieties,  or  typical  forms,  of  clouds  are  recog- 
nized in  making  meteorological  observations.  The  typical  forms  are, 


EVAPORATION,    CLOUDS,   RAIN,  AND  SNOW.  73 

however,  of  relatively  rare  occurrence.  In  noting  the  kind  of  cloud, 
the  form  is  considered  to  be  that  which  it  most  nearly  resembles.  The 
forms  are  as  follows  :  — 

1.  Cirrus.  6.  Strato-Cumulus. 

2.  Cirro-Stratus.  7.  Nimbus. 

3.  Cirro-Cumulus.  8.  Cumulus. 

4.  Cumulo-Cirrus,  or  Alto-Cumulus.  9.  Cumulo-Nimbus. 

5.  Strato-Cirrus,  or  Alto-Stratus.  10.  Stratus. 

In  the  compound  names  the  first  part  indicates  relative  height  of 
clouds. 

i>  3>  4>  6,  8,  10,  are  ordinarily  fair-weather  clouds.  2,  5,  7,  9,  are  bad- 
weather  clouds. 

HIGH    CLOUDS. 

Cirrus.  —  Cirrus  is  a  streaky,  gauzy,  wispy,  or  feathery  form  of  cloud, 
whitish  in  colour,  usually  not  very  abundant.  It  forms  at  a  great  height, 
25,000  to  50,000  feet.  It  rarely  occurs  below  16,000  feet.  It  is  some- 
times known  as  "cats'  whiskers"  and  "mares'  tails." 

Cirro-Stratus.  —  This  form  of  cloud  is  a  thin  veil  of  extended  cirrus. 
At  times  it  is  largely  composed  of  ice  particles,  as  shown  by  the  coloured 
rings,  halos,  etc.,  seen  around  the  sun  and  moon  viewed  through  it ; 
lunar  halos  are  very  frequent. 

Cirro-stratus  is  a  condensed  and  developed  form  of  cirrus  in  which 
the  streakiness  is  very  marked,  usually  on  account  of  the  greater  extent 
of  the  clouds.  At  times  ribs  of  cirro-stratus  stretch  from  a  point  on  the 
horizon  to  the  point  directly  opposite.  From  the  perspective  effect  the 
ribs  appear  widest  apart  at  the  zenith  and  converge  on  either  side. 
This  form  of  cloud  is  sometimes  popularly  known  as  "Noah's  Ark." 
The  stripes  or  ribs  are  sometimes  made  of  cross-bar  patches,  and  then 
they  are  said  to  be  striated. 

This  formation  is  also  sometimes  known  as  "polar  bands."  The 
fibres  of  cirro-stratus  sometimes  interlace  and  have  a  reticulated  appear- 
ance like  woven  cloth ;  or,  like  the  system  of  intersecting  waves  or 
breakers;  or  they  are  arranged  like  scales  of  fish,  and  are  called 
"mackerel"  sky. 


74  METEOROLOGY. 

As  waves  are  produced  on  the  surface  of  water  by  wind,  so  in  the 
air,  at  the  boundary  between  two  currents  moving  one  above  the  other, 
—  especially  if  going  in  opposite  directions,  —  there  are  very  great  waves 
which  may  be  a  mile  or  more  from  crest  to  crest.  The  rising  of  the  air 
by  the  wave  motion,  and  the  breaking  of  the  crest  as  in  the  formation 
of  foam  in  white  caps  on  the  sea,  is  possibly  the  cause  of  striated  and 
banded  forms  of  cirro-stratus.  Strong  puffs  of  wind  with  light  showers, 
alternating  during  the  day  with  clear  weather,  called  "  squally  weather," 
may  be  due  to  the  same  cause. 

INTERMEDIATE    CLOUDS. 

Cirro-Cumulus.  — •  This  is  a  dappled  or  mottled  cloud  form,  a  sheet  of 
cloud  composed  of  little  bunches  of  white  like  balls  of  cotton  wadding. 
It  consists  of  a  broken  layer  of  cloud  made  up  of  elliptical  or  elongated 
patches  of  cloud,  with  somewhat  regular  interstices,  and  without  any 
shading  of  light.  It  is  mostly  visible  near  sunset,  but  always  high  up 
in  the  sky  at  a  height  of  12,000  to  22,000  feet.  It  is  also  sometimes 
known  as  "  mackerel "  sky.  It  is  denser  than  cirrus  and  of  a  dark 
"tone." 

Cumulo-Cirrus,  or  Alto-Cumulus. — This  is  like  cirro-cumulus,  but 
composed  of  larger  bunches  of  cloud.  Sometimes  the  bunches  are 
very  compact,  with  the  edges  close  together.  It  is  also  a  high  form  of 
cumulus,  as  the  name  atto-cumulus  implies. 

Strato-Cirrus,  or  Alto-Stratus.  —  This  is  a  dense  veil  of  greyish  or 
bluish  cloud,  showing  no  optical  phenomena  such  as  coloured  rings  or 
halos  when  the  sun  or  moon  is  viewed  through  it. 

LOWER   CLOUDS. 

Strato-Cumulus.  —  This  form  is  composed  of  great  masses  of  dark 
cloud  often  covering  the  sky  completely.  It  prevails  at  a  height  of 
5000  to  10,000  feet.  It  is  essentially  a  cloud  of  the  night  and  of  the 
cold  season.  The  darker  forms  of  strato-cumulus  sometimes  give  the 
sky  an  undulatory  aspect,  especially  toward  the  horizon  an  effect  of 
perspective.  This  is  what  is  sometimes  called  "roll-cumulus."  In  high 
latitudes  in  winter,  thick  masses  of  this  cloud  obscure  the  sky  at  times 
for  weeks. 


EVAPORATION,    CLOUDS,    RAIN,   AND   SNOW.  75 

Nimbus.  —  This  is  a  dense,  thick  layer  of  dark  cloud,  without  shape 
or  form,  with  tattered  edges,  ragged  outline,  from  which  ordinarily  rain 
or  snow  continuously  falls.  Scud  is  small,  detached  masses  of  cloud,  in 
advance  or  lower  down  than  the  main  nimbus. 

Cumulus.  —  Cumulus  cloud  is  masses  of  cloud  of  a  dense,  rounded 
appearance,  like  cotton  bulging  from  a  bale.  It  is  a  heaped-up  cloud, 
arch  or  dome  shaped,  and  compact.  It  has  a  flat,  horizontal  base,  3000 
to  5000  feet  above  the  ground,  and  towers  up  conically  in  the  sky.  Its 
shape  indicates  it  is  the  result  of  the  condensation  of  moisture,  as  the 
air  in  ascending  cools  dynamically  to  the  dew-point.  Cumulus  cloud  is 
the  visible  capital  of  an  ascending  column  of  air.  It  is  essentially  a 
day  cloud.  It  is  more  common  in  the  afternoon  than  the  morning,  and 
is  most  noticeable  when  the  sun  is  low.  This  form  of  cloud  is  frequent 
at  all  times  of  the  year. 

Cumulo-Nimbus.  —  This  is  the  larger  form  of  cumulus  cloud,  the 
thunderstorm  cloud.  It  towers  up  to  a  great  height  beneath  a  layer  of 
fibrous  cloud  (false  cirrus),  at  a  height  of  about  10,000  feet,  or  much 
lower.  The  false  cirrus  is  related  physically  to  the  summit  of  the 
cumulo-nimbus.  True  cirrus  is  often  seen  above  the  false.  When  the 
rain  begins,  the  base  of  the  large  cumulus  clouds  is  seen  surrounded  by 
low,  grey,  irregular  clouds  like  nimbus.  This  is  called  the  "  cloud- 
cravat."  Ordinarily  hail  is  falling  from  it,  or  torrents  of  rain.  Above 
the  false  cirrus,  real  cirrus  cloud  is  often  seen. 

The  distant  tall  tops  of  cumulo-nimbus,  visible  near  the  horizon,  are 
known  as  "thunder-heads." 

Stratus.  —  Stratus  is  a  widely  extended  sheet  of  uniform  cloud  named 
from  its  resemblance  to  the  regular  arrangement  of  a  stratum  of  rock  or 
clay.  It  is  a  fog  lifted  up  floating  in  the  air.  It  is  the  lowest  of  all 
clouds  —  height  not  more  than  1000  to  2000  feet.  It  is  essentially  a 
night  cloud,  forming  by  radiation  of  heat  from  the  lower  layers  of  the 
air.  In  the  Polar  regions  a  singular  form  of  stratus  is  met  with.  The 
wind  raises  clouds  of  snow  to  a  height  of  15  or  20  feet,  which  envelopes 
everything  in  a  dense  fog,  as  it  were,  for  several  hours,  or  even  days,  at 
a  time.  It  is  noted  as  "moving  or  driven  snow."  Sometimes  these 
clouds  of  moving  snow  form  masses  of  great  white  clouds  for  a  certain 
distance  above  the  ground.  Ordinarily  they  are  only  in  the  lower  layers 


76  METEOROLOGY. 

of  the  air,  and  the  masts  of  ships  project  from  them  as  from  a  sea  of  fog 
or  white  smoke. 

Clouds  consist  of  minute  globules  of  water  surrounded  by  an  atmos- 
phere of  vapour  (which,  being  lighter  than  the  surrounding  air,  buoys 
them  up,  and,  farther,  prevents  the  total  evaporation  of  the  globule,  the 
atmosphere  around  it  being  saturated,  thus  giving  it  some  stability,  and 
only  dissipating  by  diffusion  when  there  is  not  much  wind).  Dines 
gives  the  diameter  of  particles  as  0.016  mm.  to  0.127  mm.  Assmann 
gives  0.006  to  0.035  mm-  They  are  at  an  average  distance  of  1.6  mm. 
from  each  other.  A  cubic  meter  of  such  a  cloud  contains  about 
262,000,000  globules.  The  weight  of  water  globules  in  a  cubic  meter  of 
cloud  is  3  grammes. 

In  the  apparent  variations  of  cloud  forms  there  is  much  that  is  fanci- 
ful, depending  on  the  way  in  which  the  light  strikes  a  cloud.  Many  of 
the  forms  and  shades  do  not  indicate  any  real  difference  in  the  mode  of 
their  formation.  A  few  isolated  clouds  go  by  one  name,  and  in  another 
case  the  same  clouds,  if  sufficient  in  amount  to  cover  a  large  part  of  the 
sky,  is  apt  to  be  called  by  a  different  name. 

Cloud  Shadows.  —  When  the  air  is  hazy,  cloud  shadows  can  sometimes 
be  traced  in  the  sky  by  dark  lines  pointing  in  the  direction  of  the  sun. 
This  is  known  as  "the  sun  drawing  water."  Similar  appearances  are 
observed  at  sunrise  and  sunset,  when  the  shadows  of  clouds  near  the 
horizon  are  projected  on  the  sky  while  the  sun  is  below  the  horizon,  and 
produce  the  effect  of  radiant  beams  diverging  from  the  sun. 

Variegated  cloud  forms  occur  mostly  in  relatively  quiet  atmosphere. 
No  important  rules  have  ever  been  formulated  as  the  result  of  the  in- 
vestigations of  cloud  forms  as  indicating  weather  changes  or  approach- 
ing storms,  and  yet  every  observer  and  predictor  makes  use  of  them. 
Clouds  mostly  indicate  regions  of  vapour  change  or  temperature  dis- 
turbance. A  balloon  will  at  times  be  seen  moving  into  a  cloud  and 
then  out  of  it,  showing  that  the  cloud  does  not  strictly  move  with  the 
air  current,  but  is  merely  a  locus  of  vapour  condensation.  The  same 
thing  is  shown  by  the  cloud-cap  around  a  mountain-top,  where  the  air 
is  continually  in  motion,  but  the  cloud  does  not  move. 

Cloud  Variation.  —  The  average  amount  of  cloudiness  is  greatest  for 
stratus  cloud  at  sunrise  all  over  the  world,  and  more  so  over  sea  than 


EVAPORATION,    CLOUDS,   RAIN,  AND  SNOW.  77 

land.  The  least  cloudiness  is  at  midday.  There  is  a  slight  increase  in 
the  afternoon,  and  a  falling  off  again  to  midnight.  The  daily  variation 
of  cloudiness,  however,  differs  in  different  parts  of  the  world.  At 
Madrid,  Spain,  the  greatest  cloudiness  for  cumulus  is  at  noon,  and  the 
least  at  night.  Over  the  sea  cloudiness  is  on  the  average  slightly 
greater  than  on  land. 

The  height  of  cumulus  clouds  increases  during  the  day  and  diminishes 
during  the  night. 

Near  the  equator,  between  the  north-east  and  south-east  trade-winds, 
the  sky  is  almost  constantly  covered  with  clouds. 

Days  are  classified  as  follows  with  regard  to  cloudiness :  Clear  days 
are  days  without  any  cloud ;  fair  days  are  days  when  there  is  some  cloud 
for  a  time,  or  the  sky  is  wholly  obscured  by  clouds  for  only  a  part  of  the 
day  ;  cloudy  days  are  days  when  the  sky  is  wholly  clouded  all  the  day. 
The  average  condition  of  cloudiness  for  all  places  over  the  whole  world 
for  the  whole  year  is  about  five-tenths  of  total  cloudiness.  The  cloudi- 
ness in  the  Lake  region  from  November  to  February  is  0.8 ;  in  summer 
it  is  0.4.  The  cloudiness  increases  in  the  winter,  in  going  north,  from 
0.3  in  Texas  to  0.6  at  the  northern  boundary  of  the  United  States.  In 
June  and  July  the  amount  of  cloudiness  is  on  the  average  the  same  over 
all  the  country  east  of  the  Mississippi,  and  about  0.4.  It  is  greater  in 
winter.  The  cloudiness  west  of  the  Mississippi  in  July  diminishes  to 
o.  i  to  the  Pacific  coast,  where  there  is  a  sudden  increase.  The  average 
cloudiness  at  any  time  of  the  year  along  the  Atlantic  coast  does 
not  seem  to  be  any  greater  than  for  places  inland.  It  is  the  least  for 
any  month  in  August.  At  Unalaska,  Alaska,  the  cloudiness  in  February 
is  nine-tenths.  At  Sacramento,  Cal.,  Keeler,  Cal.,  and  Yuma,  Ariz.,  the 
skies  are  almost  cloudless  from  June  to  September,  the  cloudiness  in 
different  years  varying  from  one-hundredth  to  nine-hundredths. 

The  number  of  sunless,  totally  cloudy  days  in  a  year  at  Jacksonville, 
Fla.,  is  85  ;  at  Washington  City,  100;  at  Boston,  m  ;  at  New  Orleans, 
73  ;  at  St.  Louis,  100 ;  at  St.  Vincent,  Minn.,  95  ;  and  at  El  Paso,  38. 
At  London,  England,  the  number  is  94,  —  50  in  the  winter  and  only 
6  in  the  summer. 

The  number  of  "  fair  "  days  in  a  year,  that  is,  days  with  cloudiness  of 
five-tenths  or  less,  but  no  rain,  is  at  Jacksonville,  Fla.,  167  ;  at  Wash- 


78  METEOROLOGY. 

ington,  143  ;  at  Boston,  135;  at  New  Orleans,  168 ;  at  St.  Louis,  140; 
at  St.  Vincent,  148;  at  El  Paso,  115.  At  New  York  City  the  average 
number  of  hours  of  sunshine  daily,  in  December,  is  5,  or  about  half  the 
possible  amount;  in  July  it  is  u  hours,  or  0.75  of  all  possible.  At 
London,  England,  the  average  duration  of  sunshine  all  the  year  round 
is  only  0.27  of  the  possible  amount,  or  3.3  hours  daily.  The  amount  is 
as  small  as  8  per  cent  in  December.  At  Madrid,  Spain,  the  sunshine  is 
60  per  cent  of  all  possible. 

FOG. 

Fog  Formation.  —  The  vapour  ascending  from  warm  water  or  warm, 
moist  ground  into  cold  air  above  it  produces  fog.  The  air  in  contact 
with  the  water  contains  more  moisture  than  the  cold  air  higher  up.  On 
account  of  its  lightness  it  ascends.  Mixing  with  the  cold  air  above,  it 
is  cooled  below  the  dew-point,  and  part  of  the  vapour  condenses  as  fog. 
It  requires  a  diminution  of  vapour  pressure  of  about  0.03  of  an  inch 
below  the  point  of  saturation  to  produce  perceptible  fog.  Fogs  form 
principally  in  the  night-time  and  usually  disappear  with  the  increasing 
temperature  after  sunrise.  Fog  consists  of  minute  drops  of  water,  and 
not  hollow  spheres  or  vesicles.  On  being  caught  on  a  glass  plate,  exam- 
ination with  a  microscope  reveals  a  hemispherical  drop,  and  not  a  ring 
of  moisture,  as  would  be  the  case  if  they  were  collapsed  hollow  spheres. 

The  ground  becomes  cooled  by  radiation,  and  the  air  in  contact  with 
it  becomes  cooled,  causing  condensation ;  the  next  air  layers  then  become 
cooled,  and  condensation  takes  place  in  them.  The  formation  of  ground- 
fog  thus  proceeds  from  the  bottom  upwards ;  its  dissipation  takes  place 
in  the  reverse  manner.  No  considerable  precipitation  occurs  as  a  result 
of  this  cooling,  as  the  continual  formation  of  a  higher  fog  layer  prevents 
the  further  cooling  of  the  lower  layers  by  radiation. 

A  warm,  damp  current  of  air  flowing  over  a  chilled  surface,  such  as  an 
ice-floe  or  a  cold  ocean-current,  gives  rise  to  a  fog.  Fogs  on  the  banks 
of  Newfoundland  are  of  this  kind.  The  warm,  moist  currents  of  air 
from  the  Gulf  Stream,  three  hundred  miles  to  the  east,  when  carried 
over  the  cold  water  of  the  banks  by  the  south-easterly  winds  of  a  low 
area  of  pressure  advancing  from  the  west,  give  rise  to  fogs.  The  tem- 
perature of  water  over  the  banks  in  July  is  45°,  while  the  Gulf  Stream 


EVAPORATION,    CLOUDS,   RAIN,   AND  SNOW.  79 

is  78°.  The  fogs  are  not  very  deep :  the  masts  of  ships  often  project 
above  them.  They  are  common  at  all  times  of  the  year,  but  more  so  in 
summer  than  winter ;  in  summer  they  prevail  about  half  the  time.  They 
are  a  serious  detriment  to  navigation  in  that  part  of  the  ocean.  The 
region  lies  in  the  shortest  path  from  New  York  to  Liverpool,  the  most 
frequented  commercial  ocean  route  in  the  world.  The  fogs  are  the 
cause  of  numerous  collisions  between  vessels,  and  between  vessels  and 
icebergs. 

The  vicinity  of  Cape  Horn,  the  region  where  the  cold  Antarctic  and 
warm  equatorial  currents  meet,  is  also  notable  for  its  fogs.  In  Behring 
Sea,  fogs  prevail  from  April  to  the  middle  of  July. 

Radiation  Fog.  —  Fog  is  formed  by  radiation  when  the  ground  is 
cooled  below  the  dew-point,  and  the  air  in  the  lowest  layers,  having 
parted  with  a  portion  of  its  moisture  as  dew,  is  still  left  in  a  saturated 
condition  and  colder  than  the  air  above  it.  If  no  mixture  with  the 
upper  air  takes  place,  as  in  the  case  of  level  ground,  usually  no  fog 
forms.  But  if  there  is  any  slope  to  the  ground,  the  colder  air  flows 
down  and  produces  a  disturbance  of  the  layers  of  air  in  the  low-lying 
parts  of  the  land,  which  gives  rise  to  a  mixture  of  the  air  and  the  pro- 
duction of  fog.  This  is  the  species  of  fog  that  rolls  slowly  down  the 
low-lying  slopes  of  mountain-sides  in  the  night-time. 

Woolly.  —  In  Alaska  this  species  of  fog  is  known  as  the  "woolly." 

Pogonip.  —  The  fog  that  forms  in  intense  cold  in  the  mountain 
regions  of  Colorado  is  called  by  the  Indian  name  pogonip.  It  begins 
in  the  valleys  and  works  up  the  mountain-sides,  contrary  to  the  usual 
mode  of  formation  of  fogs,  which  is,  to  begin  at  the  top  of  the  mountain 
and  extend  downward. 

Arctic  Fog. — There  is  almost  constantly  a  fog  in  the  Arctic  regions 
in  the  summer  months. 

On  the  north  Pacific  coast  of  the  United  States,  in  winter  time,  there 
are  heavy  fogs  preceding  the  rainy  season.  These  fogs  are  so  heavy  at 
times  as  to  produce  over  night  a  deposit  of  water  equal  to  0.05  of  an 
inch  in  depth.  Along  the  Atlantic  coast,  from  latitude  30°  to  35°,  fogs 
are  very  rare. 

Mist  is  similar  to  fog,  but  not  so  dense.  It  consists  of  streaks  and 
patches  of  fog.  The  particles  of  moisture  are  larger  than  in  fog. 


8O  METEOROLOGY. 

Dust  in  the  air  is  favourable  to  the  formation  of  fog.  All  air,  even  the 
most  transparent,  contains  myriads  of  small  floating  particles  of  dust. 

Garuas.  —  In  the  rainless  area  along  the  coast  of  Peru,  growing  plants 
are  dependent  for  moisture  on  the  heavy  fogs  that  occur  called  "garuas." 
These  prevail  from  May  to  October  after  a  summer  that  is  very  sultry. 
They  extend  over  the  country  up  to  a  height  of  1200  feet  above  the 
sea.  Above  this  level  there  is  rain. 

Iceland  Fogs.  —  At  times  dense  fogs  occur  on  the  coast  of  Iceland, 
with  disastrous  results  to  the  hay  crops.  When  large  quantities  of  field 
ice  become  stranded  along  the  shore,  it  produces  fogs  in  the  summer, 
which,  by  cutting  off  the  sunlight,  prevents  the  maturing  of  the  hay. 
The  inhabitants  know  that  when  these  fogs  set  in  the  hay  crop  will  fail. 
They  depend  very  much  on  the  work  of  their  ponies.  The  matter  is 
aggravated,  too,  by  the  fish  leaving  the  shores  at  the  same  time  on 
account  of  the  cold  water,  and  often  famine  is  the  result. 

The  fogs  that  roll  along  the  Adriatic  Sea  when  the  Bora  and  Tramon- 
tana  begin  to  blow  are  known  as  the  "fumaria"  and  the  "  spalmeggio." 

Haze.  —  The  greyish  tint  of  sky  that  is  seen  at  times,  especially  in 
summer,  is  due  to  particles  of  dust  in  the  air.  This  is  especially  marked 
in  hot  climates  during  a  prolonged  dry  spell.  The  sky  is  often  of  a  pea- 
soup  colour.  The  colour  is  in  marked  contrast  with  the  vivid  blue  of  the 
sky  after  a  heavy  rain  when  most  of  the  dust  is  washed  out. 

Indian  Summer.  —  A  condition  of  the  air  known  as  dry  fog  prevails 
sometimes  in  November  or  early  December  in  some  parts  of  the  United 
States,  that  is  called  Indian  Summer.  It  is  caused  by  dust  high  up  in 
the  air  due  to  the  smoke  of  forest  fires  and  to  some  extent,  perhaps,  by 
particles  from  the  decay  of  fallen  leaves.  This  species  of  haze  some- 
times prevails  also  in  central  Europe.  It  coincides  with  long  spells  of 
dry  weather,  and  increases  with  the  continuance  of  dry  weather,  disap- 
pearing on  the  occurrence  of  rain.  Volcanic  ashes  sometimes  cause 
these  fogs.  In  Europe,  in  1783,  such  fog  prevailed  for  a  month  by  the 
eruption  of  the  volcano  of  Hecla  in  Iceland.  A  similar  fog  prevailed  in 
1831  in  the  United  States,  in  Europe,  and  on  the  coast  of  Africa.  The 
sun  was  so  obscured  it  could  be  viewed  with  the  naked  eye  at  midday. 
The  haze  was  phosphorescent  at  night,  giving  appreciable  light. 

Callina.  —  The  "callina"  of  Spain  is  a  dry  fog.     About  a  fourth  of 


EVAPORATION,    CLOUDS,  RAIN,   AND  SNOW.  8 1 

the  sky,  from  the  horizon  up,  is  of  a  reddish-brown  tint.  High  up  the 
air  is  yellowish ;  occasionally  the  whole  sky  has  an  appearance  as  if 
covered  with  a  leaden  gauze. 

ATMOSPHERIC    ELECTRICITY. 

Electricity  of  the  Atmosphere. — The  electrical  condition  of  the  air  is 
exceedingly  variable.  The  difference  of  potential  between  the  air  and 
earth  increases  with  height  in  the  air  at  an  average  rate  of  about  3  volts 
per  foot,  but  at  a  height  of  50  feet  it  may  change  in  a  few  seconds  from 
150  to  300  volts,  or  even  more.  No  daily  period  is  perceptible  in  the 
variation  of  the  potential.  As  yet  but  few  observations  of  potential 
have  been  made.  There  are  sudden  increases  and  sharp  variations  in 
the  potential  of  the  air  in  the  vicinity  of  thunderstorms.  Possibly  this 
is  merely  induction  on  the  water-dropper.  The  electrical  conditions  of 
the  air  do  not  seem  to  have  any  simple  relation  to  each  other,  even  at 
places  close  together,  as  the  potential  changes  at  any  place  independently 
of  the  change  going  on  at  other  places  close  by.  There  is  generally  a 
fall  of  potential  of  150  to  1000  volts  occurring  from  two  to  twenty  hours 
before  the  beginning  of  rain.  A  maximum  of  wind  corresponds  to  a 
minimum  of  potential.  The  potential  rises  as  the  wind  veers  from  the 
north  to  the  south,  and  falls  as  it  backs  from  the  south  to  the  north.  It 
rises  or  falls  after  a  calm  as  the  wind  comes  up  from  a  southerly  or 
northerly  direction.  There  are  rapid  oscillations  of  potential  in  snow- 
storms. This  applies  to  Baltimore  for  a  point  in  the  air  34  feet  above 
ground  and  145  feet  above  sea  level.  Fig.  18  shows  a  simultaneous 
record  of  the  potential  of  the  air  for  a  part  of  the  day  on  July  17,  1886, 
at  Washington  at  the  Weather  Bureau  office  at  a  point  in  the  air  about 
45  feet  above  the  ground,  and  on  the  Washington  monument  at  a  height 
of  508  feet  in  the  air  and  distant  about  1000  meters  from  the  office. 
The  vertical  distance  on  the  figure  represents  volts,  and  the  correspond- 
ing horizontal  distances  the  time  of  observation.  On  that  day  the  sky 
was  cloudless,  the  atmosphere  was  hazy,  the  wind  from  the  south-west 
light,  and  no  indications  of  rain.  During  the  forenoon  the  potential  at 
the  monument  oscillated  almost  continually,  being  variable  and  high, 
often  exceeding  the  range  of  the  instrument,  which  was  3000  volts.  In 
the  afternoon  the  movements  were  less  violent,  although,  apparently,  the 


82 


METEOROLOGY. 


meteorological  conditions  remained  sensibly  the  same.  During  thunder- 
storms the  oscillations  of  potential  are  very  great.  At  the  monument, 
previous  to  a  flash  of  lightning,  the  stream  of  water  from  a  water-dropper 
collecting  the  electricity  of  the  air  is  twisted  and  split  into  many 
fine  threads  and  sprays ;  but  instantly,  with  the  occurrence  of  a  flash, 


10A.M.         11A.M.         12  M. 


1  P.M. 


2P.M. 


3P.M.       4P.M.     5P.M. 


a:  SIGNAL  OFFICE. 


&;  MONUMENT. 


FIG.  18. 


the  stream  assumes  its  normal  character,  maintaining  it  for  a  few 
seconds,  and  then  gradually  becomes  more  and  more  distorted  until 
the  occurrence  of  another  flash,  when  the  same  state  of  things  is 
repeated. 

When  rain  falls  the  potential  of  the  air  falls  to  that  of  the  earth  ; 
during  snowfall  the  potential  also  falls.     During  an  auroral  display  the 


EVAPORATION,    CLOUDS,    RAIN,    AND   SNOW.  83 

potential  of  the  air  falls  with  a  clear  sky,  indicating  possibly  invisible 
condensation  of  vapour  in  the  higher  air. 

The  electrical  condition  of  the  air  is  probably  due  in  part  to  induction 
and  in  part  to  convection,  as  when  electricity  is  carried  up  from  the  earth 
into  the  air  by  evaporation  of  water  from  the  earth's  surface,  or  by  the 
rising  of  electrified  dust  particles  in  the  air. 

In  volcanic  eruptions  there  is  often  lightning  from  the  smoke  clouds 
around  the  crater.  The  clouds  consist  of  volcanic  ashes  and  vapour 
belching  from  the  crater.  The  electric  disturbance  in  this  case  is  evi- 
dently convective.  The  ultimate  source  of  the  electricity  or  real  cause 
of  the  electric  condition  of  the  earth  and  the  air  is  problematic. 

In  the  year  1631  a  great  volume  of  smoke,  extending  160  miles, 
poured  forth  from  Mount  Vesuvius,  from  which  there  were  numerous 
electric  discharges  as  it  passed  along,  killing  cattle  and  people.  In 
another  outburst  in  1794,  the  cloud,  consisting  mostly  of  fine  ashes,  was 
borne  along  as  far  as  Tarentum,  accompanied  in  its  whole  course  by 
violent  thunderstorms  similar  to  those  that  ordinarily  occur  with  rain. 

Rainfall.  —  The  condensation  of  vapour  due  to  a  convective  mixture 
of  air  produces  an  abundant  formation  of  cloud  and  finally  rain,  where 
there  is  a  wide  contrast  of  temperature  in  the  upper  and  lower  layers 
of  the  air,  or  when  there  is  an  ascending  current.  When  rain  results 
from  an  ascending  current,  it  is  usually  very  heavy  over  a  small  area 
from  fifty  to  five  hundred  square  miles  in  extent.  Rains  from  convec- 
tive mixture,  when  the  upper  layers  of  air  have  become  relatively  cold  or 
the  lower  air  relatively  warm,  are  usually  light  and  long  continued,  and 
extend  over  great  areas  —  at  times  as  much  as  half  a  million  square 
miles. 

The  order  of  efficiency  of  the  various  causes  of  the  formation  of  rain 
are  direct  cooling  by  contact  with  colder  bodies,  or  through  radiation ; 
adiabatic  expansion,  or  expansion  with  insufficient  additions  of  heat  to 
keep  up  the  vapour  pressure ;  and  the  mixture  of  moist  masses  of  air 
having  different  temperatures. 

If  two  masses  of  saturated  air,  one  having  the  temperature  of  o°  C. 
and  the  other  20°  C.,  be  mixed,  the  greatest  amount  of  rainfall  that  is 
possible  from  the  mixture  is  0.75  grammes  per  kilogramme  of  the 
mixture.  To  obtain  this  same  amount  of  rain  from  the  warm  air  alone 


84  METEOROLOGY. 

by  adiabatic  cooling,  it  would  have  been  necessary  to  lower  the  tempera- 
ture only  i°.6  C.,  which  would  be  accomplished  by  the  air  ascending 
310  meters,  or,  if  the  warm  air  had  been  cooled  by  contact  with  a  cold 
body  or  by  radiation  only  o°.8,  the  same  amount  of  rain  would  have 
been  formed. 

Condensation  by  radiation  in  the  upper  layers  of  the  atmosphere  is 
principally  active  when  cloudiness  exists  as  a  result  of  mixture  or 
adiabatic  expansion.  It  does  occur  without  cloudiness,  though,  as 
shown  by  the  fact  of  numerous  well-attested  cases  of  rainfall  from  a 
cloudless  sky. 

It  is  hardly  possible  for  a  difference  of  temperature  to  exist  in  two 
adjacent  masses  of  air  sufficient  to  produce  appreciable  rain  by  mixture 
alone  without  some  expansion. 

It  may  be  that  at  times  there  is  such  a  condition  as  super-saturation 
before  condensation  into  rain  begins.  It  has  been  found  by  laboratory 
experiments  that  air  perfectly  free  from  dust  particles  may  become 
super-saturated  with  moisture,  which  suddenly  condenses  when  fine 
particles  are  introduced. 

The  progress  of  a  warm  current  of  air  from  the  south"  to  a  cold 
northern  region  is  favourable  to  formation  of  rain.  Winds  from  the 
ocean,  which  are  usually  moisture-laden,  are  producers  of  rain  where 
they  blow  onto  colder  land.  The  frequency  and  quantity  of  rain  at  a 
place  show  marked  dependence  on  the  direction  of  the  winds,  being 
more  frequent  and  heavier  with  some  winds  than  others.  In  the  United 
States,  winds  from  the  south-west  to  the  south-east  are  more  apt  to  be 
followed  by  rain  than  winds  from  other  directions,  at  all  times  of  the 
year.  The  region  of  the  upper  Missouri  valley  is  an  exception,  the  rains 
or  snows  in  winter  following  winds  from  the  north-west  and  north-east 
more  frequently  than  from  other  quarters. 

A  current  of  air  progressing  inland  over  a  gradually  rising  country  is 
more  apt  to  produce  rain  than  where  the  country  is  flat.  Deviations 
from  prevailing  wind-direction  are  apt  to  be  followed  by  rain.  A  moun- 
tain range,  or  even  a  range  of  hills,  when  the  air  is  near  saturation,  has 
a  powerful  effect  in  producing  rainfall  on  the  windward  side  by  causing 
the  current  of  air  to  ascend  and  to  cool  dynamically,  producing  a  tem- 
perature so  low  that  the  vapour  cannot  be  retained.  Thus  the  topograph- 


EVAPORATION,    CLOUDS,    RAIN,   AND   SNOW.  85 

ical  or  relief  features  of  a  country  exercise  a  very  great  influence 
on  the  distribution  of  rain.  High  mountain  ranges  cut  off  very 
largely  the  rainfall  on  the  leeward  side  of  the  prevalent  rain -bearing 
winds. 

Rain  and  snow  at  times  fall  from  cloudless  skies.  These  rainfalls  are 
invariably  light.  The  cloud  may  be  so  gauzy  as  to  have  no  visible 
body,  or,  between  the  time  of  its  formation  and  that  of  the  rain  reaching 
the  ground,  it  may  dissolve  and  disappear.  A  case  has  occurred  where 
such  a  shower  lasted  nearly  an  hour. 

In  the  cases  where  snows  occur  without  cloudiness,  high  winds  pre- 
vail, and  the  snow  is  probably  carried  for  a  distance  along  the  ground  or 
from  a  region  of  cloud.  Rainfalls  of  this  kind  are  of  more  frequent 
occurrence  just  after  sunset  than  at  other  times  of  the  day. 

One  of  the  conditions  that  must  precede  rain  formation  is  well  under- 
stood, but  there  are  only  few  places  where  the  component  factors  are 
sufficiently  well  known  to  permit  of  foretelling  its  occurrence.  The 
distribution  of  pressure,  wind,  and  temperature  at  the  surface  of  the 
earth  is  usually  all  that  is  shown  by  the  weather-map.  Nothing  is 
known  as  to  the  amount  of  vapour,  the  temperature,  and  the  wind 
velocity  in  the  upper  air  except  what  little  can  be  inferred  from  the 
observation  of  clouds.  In  judging  of  the  coming  of  rain,  a  knowledge 
of  the  conditions  at  a  height  in  the  air  is  as  important,  or  more  so,  than 
for  the  air  at  the  surface  of  the  earth. 

Causes  of  Rain.  —  The  conditions  of  pressure  and  temperature  at  the 
surface  of  the  ground,  which  are  seen  at  one  time  to  be  followed  by  rain, 
do  not  produce  rain  at  another  time,  evidently  from  the  absence  of  suf- 
ficient moisture  in  the  upper  air  or  a  sufficient  diminution  of  tempera- 
ture to  produce  an  upward  convection.  The  amount  of  diminution  of 
temperature  upward  not  being  known,  the  amount  of  convective  mixture 
cannot  be  inferred. 

In  by  far  the  greater  number  of  cases,  the  most  that  can  be  done 
towards  the  prediction  of  rain  from  the  weather-map  is  to  state  the 
presence  of  conditions  favourable  to  rain.  Whether  rain  will  occur  or 
not  depends  usually  on  the  presence  of  a  greater  or  less  amount  of 
moisture  in  the  air  and  the  height  to  which  it  will  ascend.  The  condi- 
tions favourable  to  rain  often  apparently  prevail  at  any  place  without 


86  METEOROLOGY. 

resulting  in  more  than  cloud  formation  at  that  locality,  but  as  the  clouds 
drift  along  rain  may  occur  at  other  places. 

Rainfall  and  Battles.  —  Much  has  been  written  on  the  connection  of 
rainfall  and  cannonading.  It  has  been  supposed  that  the  concussion  of 
artillery  fire  in  battles  produces  rain,  and  that  great  battles  are  followed 
by  heavy  rainfall.  There  is  no  reason  why  this  should  be  so.  No  phys- 
ical relation  has  ever  been  traced  between  concussion  of  air  and  forma- 
tion of  water-drops.  The  belief  is  very  ancient  that  battles  are  followed 
by  rain.  In  "  Plutarch's  Lives "  it  is  related  that  after  the  battle  of 
Marsalia  in  France  a  great  rainfall  followed,  and  it  is  mentioned  as  being 
a  well-known  fact  that  all  great  battles  are  followed  by  heavy  rain.  This 
was  certainly  a  case  where  the  rain  was  not  due  to  artillery  fire. 

Forests  have  no  effect  in  increasing  rainfall  in  their  proximity.  This 
question  is  one  to  be  relegated  to  the  infinitely  little  things  of  meteo- 
rology. 

Distribution  of  Rainfall.  — The  rainfall  in  a  year  at  the  equator  is  104 
inches  on  the  average ;  at  latitude  60°  it  is  20  inches,  or  almost  in  the 
exact  ratio  of  the  amount  of  moisture  contained  in  the  lower  air  in  the 
two  regions.  For  intermediate  latitudes,  however,  the  ratio  does  not 
hold,  more  especially  at  latitude  30°,  where  the  fall  is  only  40  inches, 
owing  possibly  to  the  general  tendency  of  the  air  in  that  latitude  to 
come  down  from  above  in  the  circulation  of  the  trade  and  anti-trade 
winds. 

All  the  vapour  in  the  air  at  any  moment,  if  condensed,  would  form  a 
layer  of  water  4  inches  in  depth  over  the  whole  surface  of  the  globe. 

The  total  quantity  of  rainfall  in  a  year  over  the  land  surface  of  the 
earth  is  estimated  at  28,000  cubic  miles  of  water,  and  averages  over  20 
inches  in  depth.  About  one-fourth  of  all  the  rainfall  runs  by  rivers  to 
the  ocean ;  the  remainder  is  evaporated  into  the  air. 

Rainfall  is  more  frequent  and  greater  on  the  sea-coast  than  inland, 
unless  there  is  a  considerable  rise  of  the  ground  in  going  into  the  inte- 
rior. On  oceanic  islands,  with  a  high  interior,  the  increase  of  rainfall 
from  the  coast  is  very  notable.  In  Scotland  along  the  divide,  about 
2500  feet  high  in  the  centre  of  the  country,  the  rainfall  in  a  year  is  over 
loo  inches,  while  along  the  coast  it  is  only  40.  On  Ben  Nevis,  Scotland, 
height  4368  feet,  the  average  rainfall  is  129.5  inches,  at  the  base  77.3 


EVAPORATION,    CLOUDS,   RAIN,   AND  SNOW.  87 

inches.  In  the  island  of  Ceylon  the  fall  on  the  coast  is  34  inches,  and 
in  the  interior,  at  a  height  of  6000  feet,  209  inches.  On  the  island  of 
St.  Helena  the  rainfall  on  the  coast  is  5.4  inches,  and  at  a  height  of  1763 
feet  24  inches. 

Rainfall  with  Height.  —  The  increase  of  rainfall  with  height  for  about 
2000  feet  is  at  the  rate  of  two-thirds  of  an  inch  for  every  100  feet,  as 
shown  by  a  20  years'  series  of  observations  in  Saxony  at  25  stations  at 
different  heights.  In  the  Tyrol  the  amount  of  rainfall  at  6000  feet  is 
greater  by  50  per  cent  than  at  2000  feet.  The  rainfall,  however,  does 
not  increase  indefinitely  with  height.  In  Hindostan  the  rainfall  is  at  its 
greatest  at  a  height  of  4000  feet,  decreasing  both  above  and  below  that 
level.  The  rainfall  on  Mount  Washington  is  83.5  inches  in  a  year,  one 
year  as  low  as  55.8  and  another  as  high  as  121.1  ;  on  Pike's  Peak  it  is 
29.4  inches,  one  year  as  low  as  9.3  and  another  as  high  as  44.6  ;  at 
Boston  the  average  fall  is  46.8  inches,  varying  from  33.8  to  65,  and  at 
Denver  14.6,  varying  from  9.5  to  20.1. 

Rain  over  Ocean.  —  Over  the  ocean  there  are  no  measurements  of 
depth  of  rainfall  of  any  consequence.  On  shipboard  merely  the  number 
of  times  rain  occurs  is  noted,  and  not  the  depth  of  rainfall.  The  rainfall 
about  the  equator  is  on  the  average  great  for  most  of  the  region.  At 
Ascension  Island,  however,  latitude  8°  south,  there  is  only  3  inches  of 
rain  in  a  year.  At  Maiden  and  other  guano  islands  in  the  Pacific  Ocean, 
from  latitude  6°  north  to  n°  south,  there  is  even  less. 

Daily  Variation.  —  There  is  a  marked  variation  in  the  amount  of 
rainfall  at  different  times  of  the  day  on  the  average.  The  greatest 
amount  falls  from  5  to  8  P.M.,  and  the  next  greatest  from  2  to  5  A.M. 
The  times  of  least  amount  of  fall  are  10  to  12  P.M.  and  8  to  10  A.M. 
The  times  of  greatest  fall  correspond  to  the  times  of  greatest  rate  of 
cooling  in  the  upper  air.  At  Batavia,  Java,  5  per  cent  of  all  the  rain- 
fall occurs  from  6  to  8  A.M.,  and  14  per  cent  from  4  to  6  P.M.  In  the 
island  of  Borneo,  with  a  rainfall  of  140  inches,  twice  as  much  falls  by 
night  as  by  day. 

Variation  in  Amount  of  Rainfall.  —  There  is  great  variation  in  the 
amount  of  rainfall  over  different  parts  of  the  earth.  The  wettest  dis- 
tricts of  the  world  are  parts  of  the  belt  of  equatorial  calms  and  certain 
localities  where  damp  winds  meet  mountain  ranges  and  are  forced 


88  METEOROLOGY. 

upward.  Cherrapunjee,  on  the  Kassia  Hills  in  Assam,  India,  has  the 
greatest  rainfall  of  any  place  in  the  world,  400  inches  a  year  on  the 
average  for  24  years.  One  year  the  amount  was  as  great  as  905  inches. 
On  the  Western  Ghauts  Mountains,  in  India,  the  annual  rainfall  is  260 
inches.  On  the  coast  of  Alaska  the  rainfall  is  1 10  inches  ;  at  Valdivia, 
in  southern  Chili,  latitude  40°  south,  116  inches.  These  places  have 
westerly  winds  blowing  over  an  extensive  tract  of  ocean,  and  their 
moisture  is  largely  thrown  down  on  the  first  coast  they  meet. 

Dry  Regions.  — To  the  north  and  south  of  the  zone  of  calms  there  is 
a  belt  of  almost  rainless  regions.  In  this  region  are  the  driest  tracts  of 
country  in  the  world,  the  Desert  of  Sahara,  in  Africa,  and  the  region 
extending  eastward  through  Arabia  to  Persia  and  further  east  to  the 
Desert  of  Gobi.  These  areas  comprise  about  5,000,000  square  miles  of 
the  surface  of  the  earth. 

In  south  Africa,  in  this  region  south  of  the  equator,  is  the  great 
Kalahari  Desert.  In  Chili  and  Peru  there  is  a  narrow  strip  of  country 
between  the  Andes  Mountains  and  the  Pacific  Ocean  where  it  rarely 
rains. 

In  the  United  States,  in  Nevada,  Utah,  southern  California,  parts  of 
Arizona  and  New  Mexico,  and  the  high  plateau  region  of  the  western 
country,  there  is  only  about  7  inches  of  rainfall  in  a  year.  Here  agri- 
culture is  carried  on  at  a  disadvantage.  Water  has  to  be  stored  in 
reservoirs  and  distributed  by  trenches  for  the  irrigation  of  growing 
crops.  In  parts  of  Spain  the  yearly  rainfall  is  only  6  inches.  At  Aden, 
Arabia,  the  rainfall  in  a  year  is  2.4  inches ;  at  Leh  Ladakh,  near  Thibet, 
2.6  inches ;  at  Petro  Alexandrowsk,  500  miles  east  of  the  Caspian  Sea, 
Asia,  2.4  inches  ;  at  Yuma,  Ariz.,  3.0  inches. 

Some  regions  of  small  and  medium  amount  of  rainfall  are  at  times 
subject  to  prolonged  periods  of  drought,  when  very  little  or  no  rain 
falls.  Such  periods  are  disastrous  to  agriculture  in  places  that  rely  on 
a  regular  rainfall. 

The  sugar  crop  of  Mauritius  Island  is  almost  exactly  proportional  to 
the  depth  of  rainfall.  Sometimes,  however,  great  excess  of  rainfall,  with- 
out sufficient  warm  weather,  makes  sugar-cane  watery  and  almost  devoid 
of  saccharine  matter.  In  California  rain  is  such  an  accurate  measure  of 
crop  yield,  an  inch  of  rain  is  said  to  be  worth  a  million  dollars. 


EVAPORATION,    CLOUDS,   RAIN,   AND  SNOW.  89 

Great  droughts  rarely  extend  over  more  than  a  single  season.  Some- 
times, however,  they  do  last  longer.  The  great  drought  in  Argentine 
Republic,  South  America,  called  "II  grand  seco,"  lasted  three  years, 
from  1828  to  1831.  Vast  herds  of  cattle  perished.  What  little  water 
there  was  in  the  few  streams  left  became  salty. 

After  a  continued  dry  spell  of  weather,  at  times  even  when  lasting 
no  more  than  two  or  three  weeks,  the  sickness  that  occurs  is  due  to  the 
drinking-water  of  springs  and  wells  being  contaminated  from  accumu- 
lated impurities  on  the  ground. 

Rainless  Region.  —  Rains  are  nearly  absent  from  the  belt  of  trade- 
winds.  The  maximum  rain  is,  in  mid-summer,  at  12°  north,  and  in 
winter  at  2°  south,  in  mid-Atlantic.  There  is  probably  no  part  of  the 
earth  where  rain  does  not  fall  occasionally.  Regions  with  less  than 
10  inches  in  a  year  are  called  "rainless."  Rain  is  a  rare  occurrence  in 
the  interior  of  Australia.  In  the  Arctic  regions,  in  Greenland,  only 

1 5  inches  fall  in  a  year ;  farther  north  there  is  only  5  inches.    On  the 
west  coast  of  Africa,  from   Cape  Town  north,  there  is  little  rain  on 
account  of  the  relatively  cool  air  from  the  cold  ocean  currents  in  the 
vicinity,  warmed  up  on  reaching  the  land  and  converted  into  dry  winds. 

Of  all  the  land  area  of  the  world,  22  per  cent  has  less  than  10  inches 
of  rainfall;  31  per  cent,  10  to  25  inches;  25  per  cent,  25  to  50  inches; 

16  per  cent,  50  to  75  inches;  6  per  cent,  over  75  inches. 
Trade-wind  Rains.  —  In  the  region  of  the  trade-winds  the  rainfall 

is  comparatively  small,  except  where  it  blows  over  a  mountain  range. 
With  the  periodical  change  in  the  trade-wind  belt,  the  dry  regions 
change  also.  This  occurs  principally  from  latitude  30°  to  40°.  Most 
of  the  rain  of  this  region  occurs  at  the  time  of  the  year  when  the  sun  is 
lowest.  This  is  the  region  of  the  winter  rains,  sometimes  called  "sub- 
tropical." It  embraces  the  lands  bordering  on  the  Mediterranean  Sea, 
the  south-eastern  part  of  the  United  States,  and,  in  the  southern  hemi- 
sphere, Cape  Colony,  Africa,  south-west  Australia,  and  New  Zealand. 
The  districts  between  these  latitudes,  on  the  eastern  sides  of  continents, 
have  a  good  deal  of  rain  in  summer  also.  This  is  the  case  in  the  east- 
ern part  of  the  United  States  and  in  China,  where  the  continental 
heating  effect  produces  easterly  winds.  In  China  the  summer  rains  are 
known  as  the  monsoon  rains.  Natal,  in  Africa,  and  the  Argentine 


90  METEOROLOGY. 

Republic,  in  South  America,  have  rains  of  the  same  character.  In  all 
these  countries  the  rains  come  at  times  most  important  for  the  growth 
of  crops ;  this  is  especially  the  case  in  the  part  of  the  United  States 
east  of  the  Mississippi  River.  These  countries,  on  this  account,  are  the 
most  favourably  situated  of  any  in  the  world  for  agriculture. 

The  amount  of  rainfall  is  different  on  the  two  sides  of  a  mountain 
range.  In  the  general  circulation  of  the  air,  the  average  or  resultant 
direction  of  the  winds  in  the  middle  and  higher  latitudes  in  the  lower 
strata  of  the  air  is  nearly  from  west  to  east.  Hence  in  the  case  of 
ranges  north  and  south,  especially  on  the  coast,  the  west  sides  in  middle 
and  higher  latitudes  are  rainy,  and  the  east  sides  dry,  and  the  reverse 
in  equatorial  and  tropical  latitudes. 

Rainfall  in  United  States.  —  In  the  United  States  east  of  the  Missis- 
sippi River,  the  average  yearly  depth  of  rainfall  diminishes  from  60 
inches  along  the  coast  of  the  Gulf  of  Mexico  and  along  the  south  Atlan- 
tic coast  to  30  inches  in  Minnesota.  Along  the  Pacific  coast  the  rainfall 
increases  northward,  from  10  inches  at  San  Diego,  Cal.,  to  50  inches  at 
Portland,  Ore.,  and  105  inches  at  Neah  Bay,  Wash.  On  the  Atlantic 
coast  it  diminishes  from  57  inches  at  Jacksonville,  Fla.,  to  44  inches  at 
New  York  City. 

Pacific  Coast  Rain.  —  Rains  on  the  Pacific  coast  occur  only  in  winter, 
and  are  mainly  due  to  the  prevailing  westerly  winds  and  the  difference 
in  temperature  over  the  land  and  sea.  In  winter  the  ocean  winds,  in 
passing  over  the  cooler  land,  lose  part  of  the  moisture  which  in  summer 
is  sustained  by  the  higher  temperature. 

In  the  following  table  is  given  the  average  rainfall  for  various  places 
in  the  United  States,  by  months  and  for  the  year,  as  derived  from  18 
years'  observations  —  from  1870  to  1888.  The  rainfalls  given  are  fairly 
representative  for  large  districts  of  country  around  the  places. 


EVAPORATION,    CLOUDS,   RAIN,   AND  SNOW. 

RAINFALL  IN  INCHES. 


PLACES. 

JAN. 

FEB. 

MAR. 

APR. 

MAY 

JUNE 

JULY 

AUG. 

SEPT. 

OCT. 

Nov. 

DEC. 

YEAR. 

ATLANTIC  COAST. 


Boston 

4.56 

3-88 

449 

3-95 

3-30 

3.58 

3-57 

4.26 

2.72 

4.12 

8.46 

3-50 

46.76 

Norfolk 

3.96 

3-82 

4.38 

4.21 

3-74 

449 

5.10 

6.30 

4.86 

3-71 

3-25 

3-92 

51-74 

Jacksonville 

3-35 

3-07 

3-8i 

3.18 

4.16 

6.66 

5.88 

7.18 

7-97 

5.80 

2.81 

3-24 

57-n 

MISSISSIPPI  VALLEY. 


St.  Paul 

1.09 

0.96 

1.49 

2.24 

3-35 

4-52 

3-28 

3-92 

343 

2.02 

!-33 

1.27 

28.9 

St.  Louis 

2.14 

2.80 

2.90 

34i 

3-97 

4-77 

3.72 

2.62 

34i 

2.84 

2.78 

2.50 

37-8 

New  Orleans 

5.69 

3-99 

5.78 

5.78 

5-15 

6.29 

6.48 

547 

4.76 

3-15 

5.10 

5.02 

62.6 

ROCKY  MOUNTAINS. 


Ft.  Benton 

0.72 

0.42 

o-54 

1-13 

2-59 

2-37 

1.65 

1.03 

o-93 

0.72 

0.58 

o-57 

13-25 

Denver 

0.69 

0-54 

0.96 

2.08 

2.64 

i-37 

'•59 

i-55 

1.02 

0.71 

0.86 

0.71 

14.72 

Ft.  Grant 

0.8  1 

1.23 

i.  06 

0.44 

0.30 

0.70 

3-34 

3-57 

1.36 

0.80 

0.72 

145 

15-78 

PACIFIC  COAST. 


Portland 

7-25 

6.83 

6.53 

347 

2.70 

1.64 

0.64 

0.63 

1.78 

3-94 

6.56 

8.31 

50.28 

San  Francisco 

4-56 

3.89 

2.83 

2.31 

o-57 

0.27 

O.O2 

O.OI 

0.15 

i.  08 

2-75 

4-59 

23-03 

San  Diego 

1.77 

2-34 

1.27 

0.96 

0-39 

0.06 

0.02 

0.16 

0.04 

045 

0.78 

1.94 

10.18 

Monthly  Variation.  —  There  are  wide  departures  from  the  average  of 
rainfalls  in  different  years.  At  Neah  Bay,  Wash.,  as  much  as  141  inches 
has  fallen  in  a  year.  In  the  lower  Mississippi  valley,  at  Baton  Rouge, 
La.,  where  the  average  fall  is  63  inches,  there  has  been  as  high  as  1 16 
inches.  At  Indio,  in  San  Diego  County,  Cal.,  in  the  region  of  the 
Mohave  Desert,  where  the  fall  is  ordinarily  2.0  inches  in  a  year,  there 
has  been  a  year  with  only  o.  i  of  an  inch. 

In  the  United  States,  except  west  of  the  Mississippi  River,  the  rain- 
fall is  quite  evenly  distributed  throughout  the  year.  On  the  Pacific 
coast,  the  summer  is  a  dry  season.  There  is  no  rain  in  July  and  August 
in  California,  except  in  the  northern  part.  In  certain  sections  of  Arizona, 
New  Mexico,  Nevada,  and  California,  there  is  rarely  any  rain  from  April 
to  October.  The  aridity  of  this  region  is  mainly  due  to  its  being  in  the 
rain  shadow  of  the  Sierra  Nevada  Mountains.  Most  of  the  moisture  is 
taken  out  of  the  air  in  ascending  and  crossing  the  range,  which  in  only 
a  few  places  is  less  than  10,000  feet  above  the  level  of  the  sea.  The 
dryness  of  the  southern  part  of  the  area  is  due,  in  part,  to  the  prevalent 


92  METEOROLOGY. 

western  current  traversing  a  relatively  cool  area  of  water  over  the 
Pacific  Ocean,  and  on  reaching  land  the  higher  temperature  prevailing 
converting  it  into  a  drying  wind. 

Over  the  country  east  of  the  Mississippi  River,  the  monthly  rainfall 
varies  from  2  to  4  inches ;  there  are,  however,  large  deviations  from 
average  values,  in  different  years.  The  rainfall  in  June  is  slightly 
greater  than  in  May,  and  usually  not  less  than  4  inches  ;  on  the  south 
Atlantic  and  Gulf  coasts  it  is  6  inches. 

Rainfalls  greater  than  10  inches  in  a  month  are  common  in  the 
United  States.  They  occur  most  frequently  from  March  to  October. 

Monthly  rainfalls  in  excess  of  10  inches  occur  occasionally.  Monthly 
rainfalls  greater  than  20  inches  occur  principally  south  of  the  Ohio 
River,  and  along  the  Pacific  coast.  There  have  been  five  monthly  rain- 
falls in  excess  of  30  inches  in  the  United  States  in  twenty  years.  In 
June,  1886,  there  was  a  fall  of  37  inches  at  Alexandria,  La.  As  much 
as  41.6  inches  has  been  known  to  fall  in  a  month  in  California. 

The  average  depth  of  rainfall  in  a  rainy  day  in  the  United  States  is 
0.2  of  an  inch. 

Most  of  the  rainfalls  that  occur  are  not  more  than  0.5  of  an  inch. 
A  fall  of  i.o  is,  however,  common.  In  most  parts  of  the  country  a  2.0- 
inch  rainfall  is  apt  to  occur.  Rainfalls  much  in  excess  of  this  occur 
occasionally.  There  have  been  reported  in  the  whole  of  the  United 
States  in  about  12  years  1506  rainfalls,  exceeding  4.0  inches  in  one  day 
from  about  1000  stations.  The  number  of  cases  were  as  follows :  4  to 
5  inches,  805  ;  5  to  6  inches,  352;  6  to  7  inches,  163;  7  to  8  inches, 
83 ;  8  to  9  inches,  35  ;  9  to  10  inches,  27;  10  to  u  inches,  20;  1 1  to 
12  inches,  6;  12  to  13  inches,  5;  13  to  14  inches,  6;  14  to  15  inches, 
2 ;  17  inches,  i  ;  22  inches,  i. 

The  greatest  measured  rainfall  in  a  day  at  any  place  in  the  United 
States  is  22.3  inches,  which  occurred  at  Alexandria,  La.,  June  15-16, 
1886.  On  the  day  preceding  there  was  a  fall  of  6.3  inches,  and  there 
had  been  previous  rainfalls  of  0.2  to  1.4  inches  for  the  seven  days  before. 
The  Red  River  rose  at  Alexandria  19  feet  in  one  day.  The  discharge 
of  water  through  the  river  showed  that  the  excessive  rainfall  occurred 
over  an  area  of  at  least  600  square  miles.  The  same  day  at  Cheney- 
ville,  22  miles  south  of  Alexandria,  the  rainfall  was  13.3  inches. 


EVAPORATION,    CLOUDS,   RAIN,  AND  SNOW.  93 

In  the  arid  regions  the  relatively  small  annual  rainfall  that  occurs 
is  often  in  the  form  of  great  downpours  or  cloudbursts  that  gully  the 
country.  A  great  part  of  the  heavy  daily  rains  in  the  United  States 
occur  west  of  the  Mississippi  River. 

In  the  tropics  very  heavy  rainfalls  occur  at  times.  At  Delhi,  India, 
there  fell,  September  26,  1875,  19.5  inches;  at  Rewah,  June  6,  1882, 
30.4  inches ;  at  Nagina  and  at  Purneah,  September  13,  1879,  32-4  and 
35.0  inches.  October  25,  1882,  30  inches  fell  at  Genoa,  Italy;  Octo- 
ber 9,  1827,  31  inches  fell  at  Joyeuse,  France. 

Duration  of  Rainfall. — The  average  duration  of  continuous  rain  in 
a  rainstorm  varies  widely  in  different  parts  of  the  world.  In  northern 
Norway  the  time  is  1 1  hours ;  in  central  Europe,  4  hours ;  in  the 
United  States  east  of  the  Mississippi  River,  5  hours ;  in  the  Rocky 
Mountains,  3.2  hours;  in  Arizona,  2.6  hours.  In  most  all  rainstorms 
three-fourths  at  least  of  all  the  rainfall  occurs  in  24  hours.  In  most 
cases  more  than  half  of  the  rain  falls  in  8  hours. 

Rate  of  Rainfall.  —  Rainfalls  at  the  rate  of  5  inches  in  an  hour  but 
lasting  only  from  fifteen  to  thirty  minutes  are  very  common  in  the 
United  States.  In  Washington  City,  July  26,  1885,  0.96  of  an  inch 
fell  in  six  minutes.  At  Galveston,  Tex.,  June  4,  1871,  3.95  inches  fell 
in  fourteen  minutes. 

Cloud-bursts.  —  A  species  of  rainfall  marked  by  an  excessive  down- 
pour of  rain  in  a  short  time  is  called  a  "cloud-burst."  These  occur 
in  the  United  States  mostly  in  hilly  or  mountainous  parts  of  the 
country  in  the  west.  The  water  pours  down  to  such  an  extent  as  to 
scoop  holes  in  the  ground  on  the  hill-sides  6  feet  deep  and  30  feet 
in  diameter.  These  rains  do  not  extend  over  a  wide  area.  They  are 
very  destructive  in  a  mountainous  country,  producing  torrents  that  carry 
everything  before  them. 

Number  of  Rainy  Days.  —  A  "  rainy  day,"  meteorologically  speaking, 
is  one  with  a  rainfall  of  o.oi  of  an  inch  or  more.  The  average  number 
of  rainy  days  in  a  year  in  latitude  43°  to  46°  is  103 ;  from  46°  to 
50°,  134;  50°  to  60°,  161. 

The  number  of  rainy  days  in  a  year  at  Jacksonville,  Fla.,  is  146;  at 
Washington  City,  rain  or  snow,  127;  at  Boston,  133  ;  at  New  Orleans, 
145;  at  St.  Louis,  116;  at  St.  Vincent,  Minn.,  96;  at  El  Paso,  Tex., 


94  METEOROLOGY. 

55.  In  the  vicinity  of  Lake  Erie  and  Ontario  the  number  is  170;  very 
many  days,  however,  with  rainfall  of  only  o.oi  of  an  inch. 

At  Toronto,  Ont,  the  average  for  30  years  is  172  days,  of  which  63 
are  days  with  snowfall.  The  greatest  number  of  rainy  days  in  June 
and  October  was  22  in  one  year,  the  average  12,  and  the  least  in  any 
year  in  the  same  months  5  each. 

Over  the  Rocky  Mountains  and  plateau  region,  the  number  of  rainy 
days  varies  from  70  to  90.  East  of  the  Mississippi  River  the  rainy  days 
are  equally  numerous  in  January  and  July,  on  the  average  about  n 
in  each ;  at  El  Paso  and  St.  Vincent  they  are  twice  as  numerous  in  July 
as  in  January.  On  the  Pacific  coast  the  number  of  rainy  days  in  a  year 
increases  from  42  at  San  Diego  to  66  at  San  Francisco,  186  at  Tatoosh, 
Id.,  224  at  Sitka,  and  250  at  Unalaska,  Alaska.  At  Keeler,  Cal.,  the 
number  is  22  in  a  year  and  at  Yuma,  Ariz.,  13. 

In  a  given  period  the  number  of  days  with  rain  divided  by  the  total 
number  of  days  in  the  period  gives  a  fractional  number  which  is  the 
probability  of  the  occurrence  of  a  rainy  day  for  the  period.  The 
probability  of  a  rainy  day  is  different  for  different  parts  of  the  country 
and  different  for  the  same  place  for  different  times  of  the  year.  It 
increases  from  o.i  in  northern  Texas  and  0.3  in  Florida,  to  0.6  in  the 
region  of  the  Great  Lakes  in  winter.  For  all  the  country  east  of  the 
Mississippi  it  is  about  0.4  in  June  and  somewhat  less  in  July.  In 
September  it  is  0.2  in  the  central  Mississippi  valley,  and  increases  to 
0.5  in  Florida  and  the  lake  region.  On  the  Pacific  coast  it  increases 
from  o.i  in  southern  California  to  0.7  in  Oregon  from  November  to 
June.  In  California,  in  June,  July,  August,  and  September,  it  is  only 
o.oi  to  0.04.  In  the  mountainous  region  of  Arizona  and  New  Mexico 
it  increases  from  0.15  in  January  and  June  to  0.4  in  July  and  August, 
and  diminishes  again  in  September. 

The  influence  of  the  prevailing  direction  of  the  wind  when  blowing 
over  water  in  increasing  the  number  of  rainy  days  is  noticeable  in  the 
lake  region.  In  January  the  number  is  12  per  cent  greater  on  the  east 
shore  of  Lake  Michigan  than  on  the  west  shore.  Toledo,  at  the  west 
end  of  Lake  Erie,  has  a  probability  of  a  rainy  day  of  0.45  as  compared 
with  0.59  at  Buffalo,  at  the  east  end. 

At  London,  England,  there  are  on  the  average  145  rainy  days  in  a 


EVAPORATION,    CLOUDS,    RAIN,   AND  SNOW.  95 

year;  on  16  to  30  of  them  the  rain  is  heavy.  The  average  number  of 
rainy  days  at  Dublin,  Ireland,  is  205  ;  the  number  with  heavy  rains  is 
1 8  to  32.  The  average  number  of  rainy  days  at  places  in  Germany  is  155. 
In  Germany  more  rains  occur  with  a  rising  than  with  a  falling  barometer. 

Snowfall.  —  When  vapour  is  condensed  at  a  temperature  below  32°  it 
freezes  and  falls  as  snow.  Depth  of  melted  snow  is  preferable  to  depth 
of  snowfall  as  a  meteorological  datum.  The  relation  between  depth  of 
snow  and  equivalent  water  depth  varies  from  \  to  -£%,  depending  on  the 
condition  of  the  snow  as  to  dryness.  When  it  is  not  practicable  to 
measure  snowfall  in  a  rain  gauge  by  melting  it,  the  measured  depth  is 
reduced  to  an  equivalent  depth  of  water  by  dividing  by  10. 

Snow  often  falls  when  the  temperature  of  the  air  is  6  or  8  degrees 
above  freezing-point.  Two-thirds  of  the  land  surface  of  the  earth  never 
has  snow.  The  lowest  latitude  in  which  snow  has  ever  been  known  to 
fall  is  Canton,  China,  in  latitude  23°,  where  on  one  occasion  it  fell  to 
the  depth  of  4  inches  (0.4  of  an  inch  melted).  Snow  occurs  everywhere 
in  the  United  States,  except  over  a  small  area  in  south-eastern  Florida. 
It  falls,  however,  only  very  rarely  along  the  California  coast  from  San 
Francisco  south,  or  on  the  Atlantic  coast  of  Florida.  In  the  southern 
hemisphere  snow  has  been  known  to  fall  at  Sydney,  Australia,  in  lati- 
tude 24°.  Snow  has  never  been  known  to  fall  at  Buenos  Ayres,  in 
latitude  34°. 5.  South  of  latitude  33°  snow  never  stays  for  any  length 
of  time  on  the  ground,  except  in  elevated  or  sheltered  spots. 

Depth  of  Snow.  —  The  average  depth  of  unmelted  snow  that  falls  in  a 
year  in  different  parts  of  the  country  is  as  follows  :  In  Maine,  8  feet ; 
in  New  York,  7  feet ;  in  Michigan,  5  feet ;  in  Iowa,  4  feet ;  in  Minne- 
sota, 3  feet ;  in  North  Dakota,  South  Dakota,  Montana,  Wyoming,  and 
Nebraska,  2  feet.  On  the  Sierra  Nevada  Mountains  the  depth  of  snow- 
fall in  a  year  ranges  from  10  to  30  feet.  Most  of  the  snow  falls  from 
December  to  March.  Usually  about  eight  great  snowstorms  occur  in  a 
year  in  New  England  and  New  York  State. 

In  the  northern  part  of  the  United  States  snow  occasionally  occurs 
in  May.  At  Quebec  snow  occurs  at  times  in  June.  At  Toronto,  Ont, 
there  have  been  three  cases  of  snow  in  June  in  30  years.  Snow  fell  at 
Lynchburg,  Va.,  June  14,  1857,  and  June  12,  1887  (height  above  sea 
level,  652  feet). 


96  METEOROLOGY. 

Forms  of  Snow.  —  Flakes  of  falling  snow  when  seen  with  a  magnify- 
ing glass  show  many  curious  and  interesting  forms  of  great  delicacy  and 
complexity.  The  figure  is  flat  and  perfectly  symmetrical,  made  up  in 
regular  geometrical  forms.  The  brilliancy  of  snow  is  due  to  the  great 
number  of  reflecting  surfaces,  arising  from  the  smallness  of  the  spiculas 
of  ice  forming  the  flake. 

Snow  is  feebly  phosphorescent ;  that  is,  it  retains  light  after  the 
source  of  light  is  withdrawn.  On  dark  nights,  when  the  ground  is  cov- 
ered with  snow,  it  appears  more  luminous  than  the  sky,  owing  to  the 
light  stored  up  during  the  day.  If  a  surface  of  snow  is  covered  with  a 
screen  on  a  bright  day,  at  night  it  is  found  to  be  less  luminous  than  the 
surrounding  snow. 

Coloured  Snow.  —  In  the  polar  regions  at  Cape  York  in  Greenland, 
where  snow  lies  unmelted  from  year  to  year,  it  acquires  a  ruddy 
colour;  occasionally  it  becomes  red  like  blood,  but  more  usually  of  a 
faint,  dull  red.  This  sometimes  occurs  also  on  the  mountains  in  south- 
ern Europe.  In  Spitzbergen  the  snow  sometimes  appears  of  a  greenish 
hue.  These  colours  are  due  to  minute  organisms. 

Gold-dust  Snow.  —  At  South  Bethlehem,  Pa.,  there  was  a  slight  fall 
of  snow  March  16,  1879,  during  the  night,  and  next  day  when  the  snow 
melted  a  yellowish  deposit  was  found  covering  the  ground.  Examina- 
tion showed  it  to  be  the  pollen  of  pine  trees.  At  Peckeloh,  Germany, 
a  yellower  " golden"  snow  once  fell  which  had  the  appearance  of  a 
surface  strewn  with  gold-dust. 

Diamond  Snow.  —  The  form  of  snow  known  in  Russia  as  "  dia- 
mond snow  "  consists  of  very  fine  particles  of  snow  suspended  in  the 
air  and  glistening  in  the  sunshine. 

Snow  Line.  —  On  high  mountains  the  snow  stays  all  the  year 
round.  The  tops  of  some  mountains,  even  within  the  tropics,  are 
always  covered  with  snow,  owing  to  the  low  temperature  at  a  great 
height.  The  limit  of  perpetual  snow,  or  the  snow  line,  is  at  the  point 
where  the  mean  temperature  of  the  air  in  the  warmest  part  of  the  year 
is  at  32°,  or  only  slightly  above  it  for  a  short  time.  This  disappearance 
of  the  snow  depends  on  the  quantity  of  the  snow  to  be  melted,  as  well 
as  on  the  duration  of  the  high  temperature.  In  middle  latitudes,  on  the 
north  side  of  a  mountain,  the  snow  line  is  sometimes  four  or  five  hun- 
dred feet  lower  than  on  the  southern  side. 


EVAPORATION,    CLOUDS,   RAIN,   AND  SNOW.  97 

Height  of  Snow  Line.  —  Near  the  equator  and  within  the  tropics 
the  snow  line  is  at  a  height  of  about  18,000  feet  above  sea  level. 
On  the  north  side  of  the  Himalayas  it  is  21,000.  In  the  Rocky 
Mountains  and  the  Caucasus  in  latitude  43°,  the  snow  line  is  at  a  height 
of  about  11,000  feet.  In  the  Alps  it  varies  from  7500  to  9000  feet.  In 
Iceland,  latitude  65°,  it  is  at  a  height  of  3000  feet ;  in  Spitzbergen,  lati- 
tude 78°,  it  corresponds  with  the  surface  of  the  earth.  The  height  of 
snow  line  varies  slightly  from  the  equator  to  latitude  20° ;  from  20°  to 
70°  it  falls  equably ;  from  70°  to  78°  it  descends  very  rapidly. 

Glaciers.  —  Snow  accumulates  to  a  great  depth  in  mountain  ravines 
in  some  places.  Under  the  great  pressure  it  becomes  compact,  solid 
ice.  These  masses  of  ice,  called  "glaciers,"  move  along  the  inclines 
of  the  valleys  like  water,  only  more  slowly.  There  is  little  or  no  evapo- 
ration from  snow  surface  except  in  the  case  of  very  dry  air. 

The  water  that  runs  from  a  melting  glacier  is  composed  of  the  part 
that  melts  by  heat  received  from  contact  with  the  air,  that  from  heat 
received  by  radiation  from  warm  objects  around,  and  the  melting  that 
results  from  the  latent  heat  of  the  vapour  condensed  out  of  the  air. 
The  last  of  these  is  more  considerable  than  the  other  two. 

In  the  Alps  there  are  about  400  glaciers  between  Mount  Blanc  and  the 
Tyrol,  covering  an  area  of  1400  square  miles.  These  follow  the  gorges 
from  the  summit  of  Mount  Blanc  down  to  a  height  of  2500  to  3500  feet 
above  the  sea.  They  move  at  a  rate  varying  in  different  places  from 
274  to  876  feet  in  a  year.  At  a  height  the  motion  is  more  rapid  than 
lower  down,  on  account  of  the  greater  steepness  of  the  incline.  In  some 
places  the  velocity  is  3  feet  a  day ;  in  winter  the  rate  of  motion  is  only 
half  as  much  as  in  summer.  The  ice  in  some  places  is  600  feet  thick. 
The  Aletsch  glacier  is  20  miles  long.  The  water  from  melting  glaciers 
forms  milky-looking  streams  holding  a  good  deal  of  pulverized  rock. 
As  glaciers  melt  they  are  continually  renewed  by  the  descending  ice 
from  above,  so  that  the  lower  end  remains  nearly  stationary,  sometimes 
advancing  a  few  hundred  feet  and  at  other  times  receding,  depending 
on  the  temperature  of  the  air  near  the  end. 

In  the  Sierra  Nevada  Mountains,  in  northern  California,  there  are  a 
number  of  glaciers  the  ends  of  which  come  down  to  a  height  of  11,000 
feet  above  the  sea.  There  are  numerous  glaciers  in  Alaska.  In  Spitz- 


98  METEOROLOGY. 

bergen  there  is  a  glacier  that  presents  a  front  of  1 1  miles  to  the  sea, 
with  a  cliff  of  ice  100  feet  high. 

In  the  Himalaya  Mountains,  in  India,  there  are  numerous  glaciers  of 
great  extent  which  come  down  to  a  height  of  9000  feet  above  the  level 
of  the  sea.  In  the  Caucasus  they  come  down  to  the  level  of  6000  feet. 

There  are  no  glaciers  in  the  tropics. 

Icebergs.  —  Almost  the  whole  of  Greenland  is  covered  with  glaciers 
that  extend  down  to  the  fiords  and  out  into  the  water  for  several  miles. 
By  the  buoyant  effect  of  the  water,  the  outer  edge  of  the  glacier  is 
lifted  and  large  masses  broken  off  from  time  to  time.  This  is  called 
"calving."  These  masses  of  ice  float  away  in  the  ocean  and  are  the 
icebergs  found  floating  in  the  lower  latitudes  sometimes  as  far  south  as 
latitude  36°.  An  iceberg  in  the  north  Atlantic  has  been  measured 
three-quarters  of  a  mile  square  and  315  feet  high  above  the  water.  The 
part  above  the  surface  of  the  ocean  is  only  about  one-ninth  of  the  part 
submerged.  The  density  of  ice,  compared  with  pure  water  at  a  temper- 
ature of  39°,  is  0.9182.  The  density  of  sea- water  is  1.0256. 

The  enormous  icebergs  of  the  southern  hemisphere  come  from  the 
glaciers  of  Victoria  Land  in  latitude  70°  to  79°  south.  One  seen  in 
latitude  37°  32'  south  measured  960  feet  in  height  and  3  miles  in  length. 
Its  great  height  indicates  there  must  be  ice  at  least  a  half  a  mile  thick 
on  the  land  from  which  it  came.  As  the  average  snowfall  in  that  lati- 
tude is  about  equal  to  10  inches  of  depth  of  water,  some  of  the  snowfall 
that  produced  the  iceberg  must  have  fallen  at  least  3000  years  ago. 

In  remote  geologic  times  glaciers  covered  a  great  part  of  the  surface 
of  the  United  States  when  the  mean  temperature  must  bave  been  much 
less  than  at  present  and  the  snowfall  much  greater.  These  glaciers 
performed  a  very  important  part  in  shaping  the  surface  of  the  ground, 
scooping  out  valleys  and  throwing  down  masses  of  rock  and  debris 
carried  from  great  distances. 

Sleet.  —  Small  pellets  of  frozen  rain  are  called  "sleet,"  and  are  formed 
by  rain  falling  through  cold  air. 

Hail.  —  Hail  consists  usually  of  a  centre  of  soft  snow  surrounded  by  a 
number  of  layers  of  alternately  transparent  and  opaque  ice.  It  is  usually 
the  size  of  a  pea,  but  often  much  larger.  Hail  is  a  rare  occurrence  at 
any  place.  It  falls  during  thunderstorms  preceded  and  followed  by  rain. 


EVAPORATION,    CLOUDS,   RAIN,   AND  SNOW.  99 

Large  hail  falls  chiefly  in  dry  hot  climates  in  summer  and  in  the  hottest 
part  of  the  day.  The  fall  of  large  hail  is  commonly  preceded  by  an 
unusual  degree  of  heat.  Large  hail  is  sometimes  irregular  in  shape. 
Sections  of  hailstones  sometimes  show  a  curious  radial  structure  due  to 
rows  of  air  bubbles  in  regular  order.  Sometimes  crystals  of  ice  are 
imbedded  in  the  body  of  the  hail.  Pyramidal  crystals  form  on  the 
exterior  at  times.  When  spherical  in  shape,  hail  has  a  tendency  to  split 
up  into  pyramids.  Very  beautiful  forms  occur  at  times,  such  as  a  cone 
with  broad  end  on  flat  side  of  a  fluted  hemisphere,  with  grooves  running 
spirally  around  the  cone  from  the  edge  to  apex.  This  form  is  evidently 
due  to  melting  produced  by  the  regular  play  of  air  currents  around  the 
hail  in  its  rotation  in  descending. 

Occurrence  of  Hail.  —  Hail  does  not  usually  fall  to  any  depth  that  is 
measurable,  consisting  mostly  of  pellets  scattered  about  on  the  ground. 
On  rare  occasions,  however,  it  does  form  a  layer  of  hailstones,  and  has 
been  known  to  be  6  inches  deep  over  limited  areas. 

Large  Hail.  —  On  August  13,  1851,  in  New  Hampshire,  during  a 
storm,  the  ground  was  covered  with  hail  to  a  depth  of  4  inches.  Some 
of  the  hailstones  were  of  great  size,  4  inches  in  diameter,  about  as  large 
as  an  orange,  and  weighed  18  ounces.  In  a  storm  that  passed  over  the 
Orkney  Islands,  Scotland,  July  24,  1818,  hail  fell  to  the  depth  of  9 
inches.  In  the  City  of  Mexico  hail  fell  to  the  depth  of  16  inches 
August  17,  1830. 

Frequency  of  Hail.  —  Hail  rarely  falls  at  the  level  of  the  sea  within 
the  tropics.  When  it  does,  the  hailstones  are  apt  to  be  of  very  great 
size.  Hail  is  more  common  at  a  height  of  1500  feet  than  at  sea  level. 
Hail  in  middle  latitudes  often  does  great  damage  to  growing  crops.  In 
France  there  are  15  hailstorms  in  a  year  on  the  average;  in  Ger- 
many, 5  ;  in  Russia,  3.  In  the  Ural  Mountains,  in  Russia,  frequent 
and  great  hailstorms  occur.  Hail  is  a  very  rare  occurrence  in  the 
British  Islands. 

Hail  in  United  States.  —  Hail  occurs  in  every  part  of  the  United 
States.  It  is  a  regular  accompaniment  of  thunderstorms,  tornadoes,  and 
cloud-bursts.  The  hailstones  are  often  of  very  great  size.  After  a  tor- 
nado cloud  passes,  and  15  minutes  after  the  rain  and  small  hail  cease, 
large  bodies  of  frozen  snow  and  ice  fall,  a  pound  or  more  in  weight. 


IOO  METEOROLOGY. 

In  the  Upper  Yellowstone  valley,  in  Montana,  after  a  cloud-burst,  hail 
fell  on  one  occasion  for  half  an  hour,  reaching  a  depth  of  14  inches, 
rooting  up  and  destroying  every  growing  thing  in  a  strip  of  country 
6  miles  wide. 

Extensive  Hailstorms.  —  Hail  usually  occurs  in  bands  or  strips  over 
the  country,  relatively  narrow  as  compared  with  their  length.  In  the 
United  States  these  bands  are  about  3  miles  wide  and  40  miles  in 
length.  On  the  I3th  of  July,  1788,  a  hailstorm  extended  from  Tou- 
raine  in  the  south-west  of  France  to  the  coast  of  Holland.  There  were 
two  strips  of  hail.  The  width  of  the  western  one  was  12  miles,  that  of  the 
eastern  one  6 ;  the  distance  between  the  strips,  on  the  average,  was  14 
miles ;  their  length  was  500  miles.  Rain  fell  outside  of  the  strips  and 
between  them. 

Formation  of  Hail.  —  Hail  forms  at  a  height  of  5000  to  16,000  feet. 
It  always  attains  its  greatest  size  below  a  height  of  5000  feet.  It  seems 
to  fall  on  the  lee-side  of  a  rise  of  ground  in  preference  to  other  places. 
Sometimes  before  the  fall  of  hail  a  peculiar  crackling  sound  is  heard  in 
the  air.  Lightning  is  always  an  attendant  of  a  hailstorm. 

Constancy  of  Climate.  —  Animal  and  vegetable  life  over  the  earth  are 
greatly  influenced  by  climate,  especially  by  the  temperature.  There  are 
various  mean  temperatures,  from  20°  to  80°,  that  bear  a  critical  relation  to 
different  kinds  of  plant  and  animal  life.  The  successful  cultivation  of 
wheat  depends  on  climate  to  some  extent,  but  not  to  such  a  great  extent 
as  generally  supposed.  The  soil  and  variety  cultivated  is  more  impor- 
tant, also  the  method  of  cultivation.  Drilled  fields  of  wheat  in  Ohio 
yield  50  per  cent  more  than  when  sown  broadcast.  The  liability  to 
destruction  by  insects  is  an  important  matter.  Poor  soils  produce  good 
grain,  but  not  a  large  yield.  A  cool,  prolonged,  and  rather  wet  spring, 
with  comparatively  light  rains  after  blossoming,  is  favourable  to  the 
crop,  with  hot  and  dry  weather  before  harvesting.  The  best  wheat 
climates  are  those  with  a  mean  annual  temperature  of  50°  to  55°. 

Climate,  or  average  weather  conditions  in  various  parts  of  the  earth, 
probably  does  not  change  in  hundreds  or  even  thousands  of  years. 
There  are  irregular  variations  from  year  to  year  in  average  temperature 
and  rainfall,  but  there  are  no  recorded  observations  that  indicate  any 
permanent  change.  The  climates  of  the  earth  are  probably  much  the 


EVAPORATION,    CLOUDS,   RAIN, : 

same  now  that  they  were  2000  years  or  more  ago.  It  is  sometimes  cited 
as  an  evidence  of  change  that  in  the  time  of  Julius  Caesar  reindeer 
abounded  in  the  forests  of  France  and  Germany,  and  that  many 
rivers  which  used  then  to  be  frozen  over  in  winter  now  remain  open. 
The  winter  of  1890-1891,  in  Europe,  was  probably  as  severe  as  any 
experienced  in  a  great  length  of  time. 

In  the  Eastern  countries  the  camel  has  entirely  replaced  as  a  domestic 
animal  the  ox  of  ancient  times,  a  change  possibly  demanded  by  the  chang- 
ing climate.  Great  changes  have  taken  place  in  Europe,  and  in  the  East 
in  the  areas  of  date,  palm,  fig,  vine,  and  olive  culture,  sometimes  supposed 
to  show  changes  of  climate.  These  do  not  prove  change  of  climate,  but 
rather  the  greater  adaptability  of  some  regions  over  others  to  certain 
plants  and  animals,  and  increased  facilities  for  transportation  of  products. 
Before  the  present  cheap  facilities  for  transportation  existed  in  Europe, 
the  vine  was  cultivated  in  regions  farther  north  than  at  present.  Culti- 
vators were  willing  to  take  the  risk  if  they  got  only  one  good  vintage 
in  six  or  eight  years.  Now  it  is  more  advantageous  to  raise  a  crop 
better  adapted  to  the  land  and  climate,  which  is  not  so  apt  to  fail, 
and  exchange  products  with  countries  farther  south  for  wine. 

Constancy  of  Rainfall.  —  There  could  scarcely  be  much  change  in 
climate  without  considerable  changes  in  rainfall.  The  Nile  in  Egypt 
is  very  much  the  same  river  it  was  2000  years  ago.  Good  evidence  that 
there  has  been  no  change  in  rainfall  is  afforded  by  the  records  of 
water  levels  in  the  landlocked  lakes  of  Italy  and  Tunis,  and  the  Dead 
Sea  in  Palestine.  These  show  no  permanent  changes  in  level.  Some- 
times the  water  is  high  for  a  number  of  years  and  then  again  low,  vary- 
ing through  wide  extremes,  but  always  returning  to  former  levels. 

The  Caspian  Sea,  which  covers  an  area  of  over  200,000  square  miles, 
is  landlocked  and  its  surface  several  feet  below  the  level  of  the  ocean. 
The  oscillations  of  its  surface  generalize  the  rainfall  over  a  very  wide 
area  of  drainage  basin.  The  level  rises  for  a  number  of  years  usually, 
and  then  falls.  Roughly,  the  period  from  high  water  to  high  water  is 
34  to  36  years.  The  records  of  levels  which  have  been  kept  for  a  long 
time  show  that  its  level  is  about  the  same  now  that  it  was  more  than 
100  years  ago. 

An  analysis  of  the  rainfall  for  321  stations  scattered  over  the  earth, 


102  METEOROLOGY. 

for  which  there  are  records  for  50  years  past,  shows  that  the  period  from 
1831  to  1840  had  a  deficiency  of  rainfall  as  compared  with  the  average, 
1846  to  1855  an  excess,  1861  to  1865  a  deficiency,  and  1876  to  1885  an 
.excess.  In  the  driest  period  the  rainfall  is  three-fourths  of  that  in  the 
rainiest.  This  variation  seems  to  be  a  real  one  for  the  whole  land  sur- 
face of  the  earth.  A  deficiency  in  one  section  is  not  counterbalanced 
by  an  excess  in  another.  There  is  no  successive  change  in  the  location 
of  a  region  of  maximum  rainfall  from  one  region  to  another  either  in 
latitude  or  longitude.  In  the  centre  of  a  continent  there  is  a  greater 
difference  between  the  amount  of  rainfall  in  a  period  of  maximum  and 
minimum  than  there  is  for  coast  regions. 

Climatic  Changes  in  Geologic  Time.  —  That  there  have  been  changes 
of  climate  in  geologic  ages,  however,  does  not  admit  of  doubt.  Evi- 
dence enough  of  this  is  afforded  in  the  fact  that  the  country  at  one  time 
was  largely  covered  with  glaciers,  and  that  again  the  country  must  have 
been  covered  with  a  tropical  luxuriance  of  vegetation  to  produce  the 
coal  fields.  These  changes  in  climate  must  have  been  exceedingly 
remote  in  time.  The  gorge  below  Niagara  Falls,  cut  by  erosion,  for  a 
distance  of  7  miles  by  the  water  of  the  cataract,  through  the  solid  rock, 
shows  that  the  waters  of  the  Great  Lakes  must  have  been  for  50,000 
years  at  least  very  much  what  they  are  at  present,  and  consequently 
that  there  has  been  no  great  change  in  rainfall  over  the  drainage  basin 
of  the  region  within  that  time. 

Ancient  Lakes.  —  Between  the  Rocky  Mountains  and  the  Sierra 
Nevada,  extending  from  Utah  to  Arizona  and  southern  California,  there 
is  an  inland  drainage  area  of  232,000  square  miles.  As  shown  by  shore 
marks  and  lacustrine  formations,  a  great  part  of  this  region  must  have 
been  in  long-past  geologic  ages  a  lake,  the  waters  of  which  flowed  to 
the  Pacific  Ocean  through  the  Columbia  River.  All  that  remains  of  it 
now  is  the  Great  Salt  Lake.  There  must  have  been  since  that  time  a 
great  change  in  climate  over  the  region.  The  yearly  rainfall  over  the 
region  at  present  is  18  inches,  and  the  possible  evaporation  80.  To 
keep  the  basin  full  and  overflowing  to  the  ocean,  the  rainfall  in  the 
region  must  have  been  much  greater  in  the  past  than  at  present,  and 
must  have  exceeded  the  evaporation. 

Constancy  of  Temperature.  —  Observations  of  temperature  with  ther- 


EVAPORATION,    CLOUDS,   RAIN,   AND  SNOW.  103 

mometers  have  only  been  made  in  comparatively  recent  times.  Ther- 
mometers graduated  arbitrarily  were  used  for  some  time  before  the 
custom  was  adopted  of  marking  the  degrees  with  reference  to  freezing 
and  boiling  point,  which  was  first  suggested  by  Fahrenheit. 

At  St.  Petersburg,  Russia,  there  is  a  record  of  temperature  since 
1743.  The  average  temperatures  for  groups  of  years  are  as  follows  :  — 

ST.  PETERSBURG.  TEMPERATURE. 

1743  to  1761 39°.o 

1762  to  1779 39°.6 

1780  to  1800 37°-9 

1801  to  1821 37°.8 

1822  to  1836 39°.2 

1836  to  1869 38°.5 

1869  to  1875 37°-9 

Mean 38°.6 

The  highest  annual  temperature,  42°. 4,  was  in  1822  ;  the  lowest,  34°.  i, 
in  1815. 

At  Paris,  France,  occasional  observations  of  air  temperature  were 
made  as  early  as  1695,  the  first  one  recorded  being  on  February  6  of 
that  year.  The  average  temperatures  for  groups  of  years  are  as 
follows :  — 

PARIS.  TEMPERATURE. 

1735  to  J74<> 5l0-2(5 

1763  to  1785 52°.i6 

1804  to  1832 5i°.26 

1833  to  1861 5i°.26 

1862  to  1871 5l0-44 

1872  to  1890 51°. 26 

The  highest  annual  temperature,  57°. 56,  was  in  1781. 

At  Vienna,  Austria,  the  average  temperatures  for  groups  of  10  and 
12  years  from  1775  to  1876  give  the  highest  average  5O°.72,  and  the 
lowest  49°.  i o.  The  highest  annual  temperature  was  54°.o  in  the  year 
1783,  and  the  lowest  46°. 6  in  1829. 


104  METEOROLOGY. 

At  Philadelphia,  Pa.,  the  average  annual  temperatures  for  various 
groups  of  years  are  as  follows :  — 

PHILADELPHIA.  TEMPERATURE. 

1758  to  1777 52°.6 

1798  to  1804 54°.2 

1829  to  1838 5i°.5 

1825  to  1845 S30-1 

1846  to  1867 54°-° 

1871  to  1889 S3°.i 

The  lowest  annual  temperature,  48°. 2,  was  in  1836. 

The  discussions  of  temperature  variation  from  long  records  over 
extensive  regions  show  no  difference  between  the  average  of  a  number 
of  years  greater  than  one  or  two  degrees. 

The  records  of  the  time  of  vintage  in  Europe  extend  back  for  several 
hundred  years.  The  records  show  groups  of  years  of  early  and  late 
vintages  corresponding  to  warm  and  dry  and  cold  and  wet  periods ; 
but  on  the  whole  there  is  no  certain  change  in  the  average  from  the 
time  of  the  oldest  .records  to  the  present. 

A  record  of  the  occurrence  of  severe  winters  has  been  kept  in  Europe 
as  far  back  as  the  year  800.  These  show,  according  to  Briickner  a  34- 
year  period  of  oscillation  from  1020  to  1190;  a  36-year  period  from 
1190  to  1370;  a  35-year  period  from  1370  to  1545;  a  34-year  period 
from  1545  to  1715,  and  a  35-year  period  from  1715  to  1890. 


CHAPTER  V. 

WINDS,  THUNDERSTORMS,  AND  TORNADOES. 

Wind.  —  The  general  circulation  of  the  air,  or  the  prevalent  wind,  is 
subject  in  many  places  to  a  daily  and  seasonal  variation,  both  in  inten- 
sity and  direction,  and  is  also  subject  to  irregular  variations,  due  to  the 
occurrence  of  storms.  There  is  in  many  cases,  no  doubt,  an  inclination 
of  the  wind  currents  to  the  ground,  so  that  the  wind  is  blowing  some- 
times slightly  up  and  at  others  down  ;  but  there  is  very  little  definitely 
known  on  this  point,  as  no  anemometer  for  measuring  the  vertical 
component  of  a  wind  has  as  yet  come  into  extensive  use. 

On  the  Eiffel  Tower  there  is  a  device  for  measuring  the  vertical 
component  of  the  wind,  consisting  of  a  vertical  tube  with  fan-wheels 
inside.  An  upward  tendency  of  the  air  is  frequently  observed,  a  down- 
ward tendency  less  often.  The  greatest  upward  inclination  observed 
has  been  nine  degrees  to  the  horizon. 

Wind  Velocity.  —  The  wind  velocity  varies  very  greatly  at  different 
places.  The  average  velocity  of  wind  in  the  United  States,  all  the  year 
round,  at  a  height  of  50  feet  above  the  ground  is  about  1 1  miles  an 
hour.  Over  the  sea  it  is  much  greater  than  at  the  same  height  over 
land.  The  difference  is  very  marked  between  a  point  on  shore  and  a 
headland  jutting  into  the  sea  or  a  lake.  At  Cape  Mendocino,  Cal.,  the 
average  velocity  is  17.4  miles  an  hour  as  compared  with  10.1  at  Fort 
Canby  in  the  vicinity.  At  Sandusky,  Ohio,  it  is  12.8  miles  as  compared 
with  8.5  miles  at  Toledo,  and  9.5  at  Cleveland.  The  velocity  at  the 
Chicago  waterworks  crib,  three  miles  out  in  Lake  Michigan,  is  twice 
as  great  as  on  the  top  of  a  high  building  in  the  city.  The  average 
velocity  increases  from  latitude  30°  to  45°.  Violent  storm  winds  in  the 
United  States  occur  2.5  times  as  frequently  from  a  northern  point  of  the 
compass  as  from  a  southern  one. 


106  METEOROLOGY. 

Increase  with  Ascent.  —  The  average  velocity  of  wind  increases 
rapidly  with  ascent  in  the  air.  On  the  Eiffel  Tower  at  1000  feet  it  is 
3.  i  times  what  it  is  60  feet  above  the  ground ;  the  ratio  is  greatest  at 
noon,  and  least  at  midnight ;  four  times  as  great  at  noon,  and  twice  as 
great  at  midnight.  On  Mount  Washington  it  is,  on  the  average,  five 
times  what  it  is  at  the  level  of  the  sea.  On  Pike's  Peak  the  average  is 
20.  i  miles  an  hour,  while  at  Denver,  5294  feet  above  sea  level,  it  is 
6.3  miles. 

Diurnal  Range  of  Wind  Velocity. —  On  the  average,  there  is  an  increase 
in  the  velocity  of  the  wind  from  the  morning  to  the  afternoon  over  the 
land.  This  range  is  small  as  compared  with  the  irregular  variations  of 
velocity  occurring.  Over  the  ocean  the  diurnal  variation  is  scarcely 
perceptible.  This  variation  is  especially  marked  when  there  are  no 
clouds.  On  cloudy  days  the  wind  velocity  is,  on  the  average,  only  half 
what  it  is  when  the  sky  is  clear.  In  some  climates  at  certain  seasons 
of  the  year  this  variation  makes  the  wind  a  veritable  storm  in  the  after- 
noon. The  wind  dies  down  toward  the  evening,  and  at  night  there  is  a 
calm.  There  is  a  very  great  increase  of  this  kind  in  the  intensity  of 
the  north-east  trade-wind  in  central  Africa  in  the  daytime.  At  Mauri- 
tius Island,  in  the  south-east  trade-wind,  the  velocity  increases  from  10 
miles  an  hour  at  three  o'clock  in  the  morning  to  18.5  miles  at  two 
o'clock  in  the  afternoon. 

Cause  of  Increase. — This  increase  of  velocity  is  due  to  the  inter- 
change of  air  above  and  below,  produced  by  the  heating  of  the  surface 
of  the  earth  and  the  much  greater  velocity  of  air  at  a  height  brought 
down  to  the  ground.  This  is  shown,  too,  by  the  fact  that  the  daily 
variation  in  the  wind  velocity  on  mountain  tops  is  the  reverse  of  what  is 
observed  at  sea  level.  The  wind  on  the  mountain  tops,  though  usually 
intense  at  most  times,  is  greatest  just  before  noon  and  diminishes  in  the 
afternoon  in  a  manner  corresponding  to  the  increase  in  velocity  at  the 
surface  of  the  earth.  The  average  velocity  on  Pike's  Peak  is  least  from 
eleven  o'clock  to  noon,  equalling  17.5  miles  an  hour,  and  greatest  from 
two  to  four  o'clock  A.M.,  when  it  is  23.2  miles  per  hour. 

The  average  time  of  greatest  wind  velocity  for  the  day  is  at  2  P.M.  in 
the  country  east  of  the  Mississippi  River,  and  the  time  of  lowest 
velocity  at  about  4  A.  M.  The  difference  between  the  average  greatest 


WINDS,  THUNDERSTORMS,  AND  TORNADOES. 

velocity  is  about  two  miles  per  hour  in  winter  and  five  miles  in  the 
summer. 

There  is,  on  the  average,  a  slight  increase  in  the  wind  velocity  during 
the  night.  The  fact  that  it  is  not  as  great  as  during  the  day  shows  that 
the  cooling  of  the  lower  air  in  the  night-time  is  due  a  good  deal  to  radia- 
tion from  the  lower  layers  of  air  as  well  as  to  the  downward  convection 
of  air  from  above. 

Daily  Variation  of  Wind  Direction.  — There  is  a  daily  variation  in  the 
direction  as  well  as  intensity  of  the  wind,  due  also  to  air  coming  from 
above,  where  the  direction  of  motion  differs  from  what  it  is  at  the 
ground.  The  amplitude  of  this  variation  is  very  different  in  different 
parts  of  the  earth,  being  complicated  with  other  variations  due  to  cur- 
rents induced  by  the  slope  of  the  ground  and  the  proximity  of  mountains 
or  water.  In  middle  latitudes,  in  the  northern  hemisphere,  the  general 
tendency  in  the  case  of  a  wind  coming  from  the  south  in  the  morning  is 
for  it  to  change  toward  west  of  south  in  the  course  of  the  day,  and  then 
back  again  to  the  south  during  the  night.  On  mountain  tops  the  change 
in  direction  is  the  reverse.  The  average  variation  is  about  12  degrees,  but 
in  some  places  it  is  much  greater.  For  instance,  at  Pola,  Austria,  at  the 
head  of  the  Adriatic  Sea,  the  wind  which  comes  from  a  direction  a  little 
south  of  east  in  the  morning  turns  to  nearly  directly  west  at  five  to  six 
P.M.,  and  then  changes  back  during  the  night. 

Land  and  Sea  Breeze.  —  The  land  and  sea  breezes  are  well-known 
periodic  winds  arising  from  the  difference  of  heating  effect  of  the  sun 
on  land  and  water.  The  wind  blows  from  the  sea  to  the  land  in  the 
daytime,  and  back  from  the  land  to  the  sea  in  the  night.  The  land  sur- 
face heats  up  more  rapidly  than  the  sea  in  the  day  and  the  air  above  it 
becomes  warmer  than  over  the  sea.  This  effect  is  reversed  in  the  night, 
the  land  radiating  and  cooling  more  than  the  water  surface.  The  sea 
cools  too,  but  the  cool  water,  by  the  greater  density  than  that  below  it, 
sinks  and  is  replaced  by  the  warmer  water  from  below. 

A  long-continued  wind  from  the  land  when  it  turns  to  come  from  the 
ocean  is  called  along  the  coast  of  New  England  a  "  sea  turn." 

Cause  of  Sea  Breeze.  — As  the  air  above  the  land  warms  up  more  than 
over  the  sea  it  expands  and  rises,  so  that  at  a  height  the  pressure  over 
the  land  is  greater  than  at  the  same  height  over  the  sea.  A  flow  of 


108  METEOROLOGY. 

air  takes  place  from  where  it  is  high  to  where  it  is  low,  which,  in  turn, 
increases  the  pressure  at  the  level  of  the  sea,  causing  a  current  to  set 
in  at  the  level  of  the  sea  toward  the  land.  The  sea  breeze  begins  out 
at  sea  and  extends  to  the  land,  as  shown  by  the  progress  of  the  ripple 
over  the  surface  when  the  sea  is  calm.  The  sea  breeze  increases  in 
intensity  during  the  day,  and  is  strongest  at  three  o'clock  in  the  after- 
noon. It  is  stronger  on  clear  than  on  cloudy  days,  and  is  more  marked  in 
summer  than  winter.  In  winter,  in  middle  latitudes,  it  is  often  masked 
by  storm  winds  and  the  currents  of  the  general  circulation  of  the  air. 

Sea  Breeze  First  Visible.  —  Considering  the  cause  of  the  sea  breeze, 
it  would  seem  that  in  the  case  of  a  flat  shore  and  deep  water  the 
ripple  ought  first  to  be  observed  at  the  shore  and  extend  out  gradually 
into  the  sea ;  for  it  is  just  at  the  shore  that  the  contrast  or  variation  of 
surface  temperature  is  greatest  and  the  difference  of  pressure  conse- 
quently the  greatest.  But  the  wind  in  a  short  distance  is  so  light  that 
it  is  not  noticeable  or  capable  of  stirring  the  water.  It  requires  a  con- 
siderable width  of  land  and  water  to  produce  a  cumulative  effect  suffi- 
cient to  make  an  appreciable  ripple,  and  therefore  it  is  usually  observed 
first  some  distance  from  the  shore.  In  the  case  of  a  slopping  shore  or 
steep  mountains  back  from  the  shore,  the  interchange  of  air  may  first 
commence  between  the  slopes  and  the  air  out  some  distance  to  sea,  in 
which  case  the  ripple  is  naturally  first  observed  in  the  offing  coming 
towards  the  land.  The  air  over  the  land,  when  heated,  expands  side- 
ways as  well  as  upward,  and  causes  a  tendency  to  an  outward  current 
which  counterbalances  for  a  time  the  tendency  of  the  air  to  flow  from 
where  it  is  denser.  This  may  be,  in  part,  the  explanation  of  the  ripple 
being  first  seen  out  to  sea. 

Regularity  of  Sea  Breeze.  —  The  land  and  sea  breeze  blows  with  great 
regularity  in  the  tropics,  except  where  masked  by  other  stronger 
periodical  winds,  as  in  the  case  of  the  monsoon  winds  in  India.  In  the 
sultry  climates  of  the  tropics,  the  coming  of  the  sea  breeze,  which 
usually  sets  in  about  ten  o'clock  in  the  morning,  is  awaited  impatiently 
by  the  inhabitants  every  day.  With  its  appearance,  the  oppressive 
sultriness  is  relieved,  and  the  refreshing  air  brings  new  life  not  only 
because  of  its  relative  coolness,  but  also  by  replacing  the  stagnant 
malarious  air  of  the  land. 


WINDS,    THUNDERSTORMS,    AND    TORNADOES.  1 09 

Strong  Sea  Breeze.  —  Where  the  sea  breeze  has  the  same  direction  as 
the  prevailing  circulation  of  the  air,  in  some  places  it  increases  to  such 
an  extent  during  the  day  as  to  become  a  storm  wind.  In  the  summer 
of  the  southern  hemisphere  the  sea  breeze  at  Valparaiso,  Chili,  is  very 
strong.  It  blows  regularly  in  the  afternoons  with  such  violence  that 
pedestrians  have  to  seek  shelter.  Public  places  are  deserted  and 
business  suspended.  Communication  between  ships  and  the  shore  is 
cut  off.  The  sky  is  without  a  cloud  and  the  atmosphere  perfectly 
transparent.  This  occurs  day  after  day  with  the  greatest  regularity. 
Late  in  the  afternoon  the  wind  quiets  down  very  suddenly.  The 
intensity  of  the  sea  breeze  in  this  particular  case  is  augmented  by  the 
specially  low  temperature  of  the  sea  in  the  vicinity  of  the  shore,  by 
the  great  heating  effect  of  the  sun  on  the  dry,  barren  land  and  the  moun- 
tain sides,  and  by  the  prevailing  direction  of  the  general  current  of  the 
air,  which  is  from  the  south-west.  At  Kingston,  Jamaica  Island,  the 
sea  breeze  is  almost  as  strong  as  at  Valparaiso.  The  intensity  of 
the  wind  is  due  to  the  Blue  Mountains  in  the  vicinity. 

The  differences  of  pressure  associated  with  the  land  and  sea  breeze 
are  very  small.  Still,  they  are  perceptible  in  the  differences  of  mean 
pressures  for  a  number  of  years  at  coast  and  inland  stations.  In 
England  the  pressures  at  coast  stations  from  10  A.M.  to  n  P.M.  are 
perceptibly  higher  than  at  places  inland.  The  heating  and  cooling  are 
of  such  short  periods  that  the  winds  do  not  reach  any  great  develop- 
ment and  extend  only  a  few  miles. 

Kona.  —  In  the  Hawaiian  Islands  the  interruption  of  the  north-east 
trade-wind  by  a  wind  from  the  south-west  is  usually  associated  with  rain. 
This  occurs  principally  from  December  to  April.  The  wind  is  called 
the  "Kona." 

Monsoon.  —  The  monsoon  winds  of  Asia,  from  a  word  meaning 
season,  arise  from  the  same  cause  as  the  land  and  sea  breeze,  but  are 
due  to  seasonal  differences  in  temperature  effects  over  the  continent  of 
Asia  and  the  Indian  and  Pacific  oceans.  The  land  warms  up  more  in 
the  summer  than  the  sea,  and  in  the  winter  cools  off  more.  This  gives 
rise  to  south-west  winds  from  April  to  October  over  a  district  extending 
from  Australia  to  India,  completely  breaking  up  the  north-east  trade- 
winds  of  the  region.  The  wind  then  changes  direction,  and  from  Octo- 


HO  METEOROLOGY. 

her  to  April  blows  from  the  north-east.  These  are  the  famous  summer 
and  winter  monsoons.  The  same  differences  of  pressure  exist  as  in  the 
case  of  the  land  and  sea  breeze,  only  greater.  In  July  the  pressure  over 
central  Asia  is  29.6  inches,  and  over  the  Indian  Ocean  29.9.  In  January 
the  pressure  over  Asia  is  30.4  inches,  and  over  the  Indian  Ocean  30.0. 

Intensity  of  Monsoon. — The  intensity  of  the  south-west  monsoon 
in  India  is  greatly  increased  by  the  diminution  of  pressure  over  the 
land,  produced  by  the  collapse  of  the  great  quantity  of  vapour  taken  from 
the  air  in  rising  and  crossing  the  Himalaya  Mountains  to  the  north 
of  India.  This  produces  the  tremendous  downpours  of  rain  for  which 
this  region  is  noted,  the  greatest  that  occur  anywhere  in  the  world. 

Differences  of  Wind  Direction.  —  The  direction  of  the  wind  is  not 
everywhere  south-west  in  the  summer  monsoon.  The  general  tendency 
of  the  air  is  toward  the  centre  of  the  continent  of  Asia.  In  some  places 
the  direction  is  locally  modified.  Along  the  coast  of  China  the  direc- 
tion of  the  wind  is  southerly ;  more  to  the  north  it  is  from  the  east. 
In  the  arctic  regions  north  of  Asia,  even,  the  monsoon  effect  is  visible, 
but  very  weak ;  the  direction  is  from  the  north-west.  The  effect  extends 
south  of  the  equator,  whence  air  is  drawn,  thereby  intensifying  the 
south-east  trade-winds. 

There  is  no  belt  of  calms  in  the  Indian  Ocean  during  the  summer 
monsoon. 

The  cooling  of  the  air  over  Australia  while  the  air  over  Asia  is  being 
heated  also  adds  to  the  monsoon  effect. 

Bursting  of  the  Monsoon.  —  The  turning  of  the  monsoon  current  is 
called  the  "  bursting  "  of  the  monsoon.  It  does  not  occur  all  at  once 
or  in  a  day.  Sometimes  for  as  much  as  three  or  four  weeks  at  the 
epoch  of  change,  the  winds  are  feeble  and  uncertain  in  direction.  At 
this  time  violent  storms  occur,  especially  when  the  "bursting"  of  the 
south-west  monsoon  is  delayed.  The  wind  finally  settles  to  the  south- 
west. After  a  few  days  the  crashing  sea  waves  along  the  shores  tell 
that  the  monsoon  is  advancing.  The  lightning  flashes,  the  thunder 
roars,  and  the  rain  comes  down  in  torrents.  The  rivers  rise  thirty  feet 
in  a  single  night.  The  bursting  of  the  monsoon  lasts  with  varying 
intensity  for  three  or  four  weeks.  During  this  time  it  rains  almost 
incessantly.  Then  it  clears  and  the  wind  blows  strong  and  steady  from 


WINDS,    THUNDERSTORMS,   AND    TORNADOES.  Ill 

the  south-west  for  the  next  five  months.  Rain  occurs  more  or  less  all 
through  this  monsoon,  which  is  called  the  wet  monsoon  as  distinguished 
from  the  dry  monsoon  which  blows  from  the  north-east.  This  applies 
to  Ceylon  and  southern  India. 

Mountain  Winds.  — There  is  a  class  of  winds  peculiar  to  mountainous 
regions,  that  blow  with  notable  regularity.  They  are  due  to  the  heat- 
ing on  the  inclined  surface  of  the  mountain.  When  not  modified  by 
the  stronger  winds  of  the  general  air  circulation,  they  are  up  the 
mountain  in  the  day  and  down  in  the  night. 

Cause  of  Wind.  —  On  the  side  of  a  mountain,  as  shown  in  Fig.  19, 
when  the  air  is  calm,  the  surfaces  of  equal  pressure  are  horizontal  as 


FIG.  19. 

in  the  case  of  the  line  bd.  When  the  sun  shines  on  the  mountain 
side,  the  column  of  air,  ab,  expands  to  the  point  c,  raising  the  pressure 
at  b  to  the  point  c,  while  at  d  it  remains  unchanged.  There  is  con- 
sequently a  flow  of  air  from  c  to  </,  to  restore  the  equilibrium.  The 
consequence  is  there  is  a  wind  blowing  up  the  mountain  during  the  day. 
In  the  cooling  during  the  night  this  process  is  reversed,  and  the  wind 
is  down  the  mountain.  Hunters  make  camp-fires  below  their  tents,  so 
that  the  night  wind  may  carry  the  smoke  away. 

The  shapes  of  valleys  modify  these  winds  at  different  times  of  the 
day  and  at  different  seasons  of  the  year.  Sometimes  the  wind  is 
stronger  by  day  than  night  and  again  the  case  is  reversed.  In  winter 
a  snow  covering  is  favourable  to  the  night  wind;  in  summer  the  heating 
effect  of  the  sun  in  the  day  is  favourable  to  the  day  wind.  Where  the 
air  is  brought  together  by  a  number  of  ravines  into  one  contracted 
current,  its  force  may  become  very  great. 

Names  of  Mountain  Winds.  —  These  winds  are  given  names  in  some 


112  METEOROLOGY. 

localities.  On  Lake  Como,  in  Italy,  the  day  wind  is  called  "La  Breva" 
and  its  opposite,  the  night  wind,  the  "Tivano." 

Any  interruptions  to  these  winds  in  countries  where  they  blow 
regularly  is  very  noticeable,  and  indicates  some  powerful  disturbing 
cause,  usually  the  proximity  of  a  storm. 

In  some  places  only  the  night  wind  is  noticed,  being  felt  by  its  lower 
temperature.  This  is  the  case  in  narrow  gorge-like  valleys  that  connect 
with  broader  more  strongly  heated  valleys. 

The  side  of  a  mountain  in  shadow  in  the  daytime  may  become  colder 
than  the  surrounding  air  and  send  down  an  avalanche  of  cold  air  on  the 
warm  valley  below. 

Williwaus.  —  A  most  interesting  climatic  feature  of  Terra  del  Fuego, 
Patagonia,  is  the  violent  blast  of  wind  called  the  "Williwaus,"  which 
at  times  comes  down  into  the  fiords  from  the  mountain  sides.  It  is 
of  the  most  frightful  intensity.  A  deep  muttering  is  heard  in  the 
distance.  Suddenly,  without  any  gradation,  a  most  terrible  blast  comes 
down  upon  the  sea.  Immediately  the  water  is  pulverized  and  scattered 
in  fine  drops,  and  driven  through  the  air  by  the  incredible  force  of  the 
wind  which  resembles  a  hurricane.  The  shock  lasts  from  8  to  10 
seconds,  and  a  calm  follows  as  suddenly  as  it  was  interrupted.  A  ship 
at  anchor  has  about  time  to  stretch  its  cable.  No  vessel  could  stand 
the  wind  for  more  than  a  few  seconds.  The  barometric  pressure  sud- 
denly increases  0.08  of  an  inch. 

Avalanche  Winds.  —  Avalanches  in  the  Alps  and  other  mountains  — 
that  is,  the  sliding  down  of  great  masses  of  snow,  carrying  with  them 
rock  and  earth  —  produce  winds  of  great  intensity  over  restricted  areas. 
Persons  in  the  blast  of  an  avalanche  have  their  clothes  torn  to  shreds  by 
the  violence  of  the  wind.  The  extent  of  an  avalanche  is  sometimes  as 
great  as  360,000,000  cubic  feet  of  material,  which  falls  at  times  3000 
feet  or  more. 

Cape  Town  Wind.  —  A  peculiarly  violent  wind  blows  down  from 
Table  Mountain,  opposite  Cape  Town,  in  South  Africa.  A  dense  mantle 
of  vapour  rests  upon  the  mountain  and,  when  the  wind  begins,  pours 
over  its  steep  sides  like  a  cataract  of  foam,  suggesting  the  idea  of  a 
tablecloth.  The  storm  wind  is  known  locally  as  the  "  clearing  of  the 
tablecloth." 


WINDS,    THUNDERSTORMS,   AND    TORNADOES.  113 

Fohn-wind.  —  There  is  a  peculiar  class  of  mountain  winds  associated 
with  the  movement  of  areas  of  low  barometric  pressure  over  the  surface 
of  the  earth.  A  wind  of  this  kind  is  the  Fohn-wind  of  Switzerland. 
It  is  warm  and  very  dry  and  comes  down  over  the  mountains  with 
great  violence.  It  blows  from  the  south-east  and  south.  Its  direction 

o 

depends  on  the  valleys  through  which  it  comes.  The  intensity  of  the 
vvind  diminishes  with  distance  from  the  mountains.  It  is  the  southerly 
current  of  air  moving  from  Italy  toward  a  centre  of  low  pressure  north 
of  the  Alps. 

Cause  of  Fohn.  —  As  the  air  ascends  the  mountains,  it  is  cooled 
dynamically  by  expansion.  This  cooling  condenses  the  vapour,  and  there 
is  rain  on  the  side  of  the  mountain  from  which  the  wind  comes.  The 
latent  heat  in  the  vapour  is  set  free.  The  air  in  descending  the  moun- 
tain on  the  other  side  is  dynamically  heated  by  compression,  Air  going 
over  without  any  condensation  of  vapour  would  remain  unaltered  in  tem- 
perature, for  it  would  have  been  warmed  in  the  descent  just  as  much  as 
it  had  been  cooled  in  rising.  But  when  there  is  condensation  of  vapour 
the  latent  heat  is  added,  so  that  it  is  much  warmer  after  its  descent 
than  before.  The  dynamic  heating  effect  is  0.58  of  a  degree  Fahren- 
heit for  every  hundred  feet  of  descent.  The  potential  temperature  of 
the  higher  layers  of  the  atmosphere  is,  in  general,  higher  than  that  of 
the  lower.  The  potential  temperature  increases  when  condensation 
occurs,  and  the  increase  is  greater  the  more  water  the  air  loses.  The 
warmth  of  the  Fohn  when  it  first  blows,  especially  in  winter,  is  not 
due  to  latent  heat  of  condensing  vapour,  but  simply  to  dynamic 
warming  in  descent.  The  upward  rate  of  diminution  being  less  than 
the  adiabatic  rate,  there  is  relative  warming  in  the  case  of  descending 
air. 

Prevalence  of  Fohn.  — The  Fohn  blows  with  varying  violence  for  two 
or  three  days,  breaking  trees  and  overturning  houses  —  creating  great 
commotion  and  uproar  and  causing  terror  to  the  country.  The  dreaded 
avalanches  are  coincident  with  its  appearance.  People  suffer  under  the 
influence  of  the  hot  wind  ;  it  oppresses  the  spirit  and  is  a  strain  on  the 
nervous  system.  Domestic  fires  are  extinguished  under  the  supervision 
of  a  fire-patrol  from  house  to  house,  so  great  is  the  dread  of  a  conflagra- 
tion which  would  be  disastrous  in  the  thoroughly  dried  condition  of  the 


114  METEOROLOGY. 

timber  in  the  houses,  due  to  the  dryness  of  the  air.  After  the  preva- 
lence of  the  Fohn,  there  are  usually  heavy  rainfalls. 

Dryness  of  Fohn.  —  The  Fohn  occurs  occasionally  from  November  to 
March,  there  being  about  30  days  of  it  on  the  average.  The  tempera- 
ture during  its  prevalence  is  about  30  degrees  above  the  average  for  the 
time  of  the  year.  The  moisture  in  the  air  is  not  more  than  one-seventh 
of  what  it  ordinarily  holds  and  only  one-tenth  of  what  it  is  capable  of 
holding.  The  Fohn  in  the  spring  is  welcomed,  for  it  quickly  disposes 
of  the  winter  accumulation  of  snow  and  ice.  In  the  Grindelwald  valley 
it  takes  away  2  feet  of  snow  in  12  hours,  and  does  as  much  in  a  day 
toward  clearing  away  ice  as  two  weeks  of  ordinary  spring  weather. 

Chinook.  —  The  Chinook  winds  in  the  western  part  of  the  United 
States  are  of  the  same  nature  as  the  Fohn  winds  of  Switzerland.  They 
occur  from  the  southern  part  of  Colorado  up  into  British  America,  prin- 
cipally in  Wyoming  and  Montana.  They  occur  as  far  up  as  the  Arctic 
Circle.  They  soften  the  winter  climate  materially.  They  are  warm, 
dry,  westerly,  or  northerly  winds  on  the  eastern  slopes  of  the  Rocky 
Mountains,  sometimes  lasting  for  several  days,  and  sometimes  only  a 
few  hours.  They  occur  when  areas  of  low  pressure  pass  along  north  of 
the  places.  The  high  temperatures  are  confined  to  the  valleys,  occur- 
ring in  streaks.  A  person  will  sometimes  pass  from  a  very  warm  to  a 
very  cold  atmosphere  in  crossing  a  valley. 

Mistral.  —  The  circulation  of  air  induced  by  high  and  low  areas  of 
pressure  sometimes  carries  cold  bodies  of  air  from  high  plateaus  down 
onto  warm  regions.  These  winds  in  some  places  are  given  names. 
The  "  Mistral "  is  a  wind  of  this  kind  which  blows  in  the  Rhone  valley 
in  the  south-eastern  part  of  France. 

Bora.  — The  "  Bora  "  of  the  Adriatic  Sea  is  a  wind  which  blows  down 
off  the  high  plateau  of  Carinthia.  At  Trieste,  Austria,  it  is  a  furious 
northerly  wind. 

Tramontana.  —  On  the  Italian  side  of  the  Adriatic  Sea,  the  same 
wind  which  blows  from  the  mountain  along  the  shore  is  called  the 
"Tramontana." 

Gregale.  —  At  the  island  of  Malta  the  same  wind  is  known  as  the 
dreaded  "  Gregale." 

Buran. — The  steppes  and  deserts  of  central  Asia  are  subject  to  a 


WINDS,    THUNDERSTORMS,   AND    TORNADOES.  115 

north-east  wind  called  the  "  Buran,"  which  blows  as  a  gale.  It  is  very 
cold  and  carries  clouds  of  drifting  snow. 

Purga. — The  "  Purga  "  is  another  name  for  a  more  violent  wind  in 
Siberia,  of  the  same  kind  as  the  Buran. 

Blizzard.  —  In  the  United  States  the  north  wind  in  Montana  and  the 
Dakotas  in  winter  is  at  times  very  strong  and  exceedingly  cold.  This 
wind  is  called  the  "Blizzard."  In  the  most  violent  of  these,  the  wind 
will  blow  at  the  rate  of  50  miles  an  hour  for  a  whole  day,  with  the  tem- 
perature 30°  below  zero.  In  these  storms  the  wind  has  been  known  to 
blow  at  the  rate  of  over  40  miles  an  hour  for  100  consecutive  hours. 
No  one  exposed  to  these  winds  can  live  for  any  great  length  of  time. 
Low  temperatures  can  be  borne  without  much  inconvenience  when  the 
air  is  still.  The  body  creates  about  it  a  locally  warm  atmosphere. 
When,  however,  there  is  any  wind,  this  atmosphere  is  carried  away  and 
has  to  be  perpetually  renewed,  causing  a  great  drain  of  heat  from  the 
system.  In  the  arctic  regions  in  quiet  air,  with  the  temperature  40° 
below  zero,  a  person  can  be  out-doors  and  take  pencil  notes  with 
ungloved  hands.  But  in  a  wind  of  40  miles  an  hour  even  a  temperature 
of  freezing  would  be  unendurable  for  any  length  of  time. 

The  blizzard  extends  from  Texas  to  Illinois,  but  is  milder  than  to  the 
west  and  north-west. 

Northers.  —  From  Missouri  to  the  Gulf  of  Mexico  the  north  winds 
of  winter  are  known  as  "  Northers."  In  Texas  they  are  distinguished 
as  dry  and  wet  northers,  depending  on  rain  or  its  absence.  There  is 
less  likelihood  of  their  being  wet  the  farther  the  low  areas  they  are 
associated  with  are  from  the  Gulf  of  Mexico. 

Barber.  —  In  the  Gulf  of  St.  Lawrence  a  strong  wind  blows  at  times, 
loaded  with  particles  of  frozen  fog.  Driven  by  the  high  wind,  it  almost 
cuts  the  face.  This  wind  is  called  the  "  Barber." 

Pamperos.  —  In  southern  Brazil,  the  Argentine  Republic,  and  Uraguay 
the  south-west  winds  which  correspond  to  the  "Northers"  of  the  Gulf 
of  Mexico  are  called  "  Pamperos,"  from  the  Spanish  word  for  a  plain. 

Southerly  Buster  Nor'wester. — In  New  Zealand  the  southerly  winds 
of  the  same  nature  as  the  "  Northers  "  of  the  northern  hemisphere  are 
called  the  "  Southerly  Buster  "  and  the  "  Nor' wester." 

Scirocco.  —  The  difference  in  heating  effect  over  level  ground,  such 


Il6  METEOROLOGY. 

as  over  desert  and  cultivated  land,  gives  rise  to  winds  of  marked  pecu- 
liarity, but  no  great  violence.  On  the  African  coast  of  the  Mediterra- 
nean Sea  and  in  Malta  and  Italy,  there  is  a  south-east  wind  in  summer 
called  the  "  Scirocco."  It  is  a  hot  and  dry  wind,  and  produces  a  feel- 
ing of  lassitude.  It  comes  from  the  moderately  high  lands  of  Africa. 
In  the  Island  of  Sicily  it  sometimes  brings  a  temperature  of  1 10°. 

La  Veche.  —  It  reaches  as  far  as  Spain  at  times,  where  it  is  called 
"  La  Veche." 

Leste.  —  In  the  north  of  Africa  it  is  called  the  "  Leste." 

Desert  Winds.  —  The  easterly  winds  which  blow  on  rare  occasions  in 
southern  California  in  summer  from  the  interior  of  the  country  toward 
the  Pacific  Ocean  are  called  "  Desert  Winds."  The  temperature  in 
these  winds  sometimes  goes  very  high,  120°  or  more.  During  their 
prevalence  shelter  has  to  be  sought  by  persons  in  the  closed  adobe 
houses  or  cool  cellars.  Persons  in  small  boats  on  the  ocean  near  the 
coast  have  lost  their  lives  in  these  winds,  not  being  able  to  reach  the 
shore  and  shelter  before  exhaustion  from  the  scorching  heat.  This 
wind  lasts  but  a  few  hours. 

There  are  desert  winds  that  blow  from  the  bad  lands- in  Dakota, 
but  of  no  great  violence. 

The  hot  winds  of  Kansas  that  blow  in  summer  from  the  south-west 
are  very  dry,  and  parch  the  growing  crops.  In  the  hot  winds  of  Kansas 
the  air  of  excessively  high  temperature  comes  in  streaks  or  patches,  and 
blows  along,  lasting  three  to  five  minutes  and  in  some  cases  even  as 
much  as  half  an  hour,  when  there  is  a  return  to  the  ordinary  tempera- 
ture, and  perhaps  again  a  renewal  of  the  warm  stream.  The  warm  cur- 
rents prevail  in  narrow  currents  100  to  500  feet  wide,  and  the  air,  besides 
being  very  warm,  is  very  dry.  It  withers  growing  vegetation.  Some- 
times the  leaves  of  trees  are  so  withered  and  dried  as  to  fall  to  powder 
in  one  of  these  hot  blasts.  Associated  with  these  hot  winds  of  Kansas 
there  are  V-shaped  areas  of  barometric  depression,  extending  from  north 
to  south  from  Nebraska  to  Manitoba.  The  winds  are  due  to  descending 
currents  of  air,  which  are  heated  dynamically  by  compression  in  coming 
down.  They  occur  in  July  and  August  from  western  Texas  north  to 
Kansas,  and  mostly  after  great  rainfalls. 

Harmattan.  —  On  the  west  coast  of  Africa  there  is  a  hot  east  wind 


WINDS,  THUNDERSTORMS,  AND  TORNADOES.         1 1/ 

called  the  "Harmattan."  Coming  off  the  desert,  it  brings  with  it 
clouds  of  dust  smutty  red  and  white  or  copper  coloured,  which  covers 
the  sails' and  decks  of  ships  far  out  to  sea,  and  which  is  largely  com- 
posed of  the  microscopic  shells  of  infusoria. 

Khamsin.  —  In  Egypt  the  hot  wind  from  the  desert  is  known  as  the 
"  Khamsin  "  or  "fifty,"  from  the  prevalent  notion  that  it  blows  for  that 
number  of  days.  It  is  at  times  a  pestilence-bringing  wind.  When  this 
wind  begins  to  blow  the  natives  gather  in  the  churches,  and  a  general 
burial  service  is  performed.  Any  one  dying  during  the  prevalence  of 
the  wind  is  buried  without  further  ceremony. 

Simoon.  —  The  "  Simoon  "  is  a  sand  whirlwind  occurring  in  the 
desert.  It  sometimes  overwhelms  and  buries  whole  caravans.  Light- 
ning occurs  with  the  simoon  (due  to  development  of  electricity  by 
friction  of  sand  on  the  air). 

For  the  most  part,  the  names  given  to  winds  do  not  signify  any  differ- 
ence in  their  physical  causes.  The  names  originate  from  their  effects 
on  human  beings  and  the  different  feelings  aroused.  The  searching 
wind  of  the  blizzard  makes  one  feel  as  if  pursued  by  a  demon.  The 
hot  scorching  blast  of  the  simoon  and  the  mysterious  sand-column 
of  the  desert,  wandering  in  its  burning  solitude,  inspire  awe  and  fear. 
The  tendency  to  personify  nature  is  strong  in  the  human  mind.  Expos- 
ure to  the  inconvenience  and  suffering  caused  by  wind  makes  a  deep 
impression. 

Wind  Roses.  —  Cloudiness,  rainfall,  high  or  low  temperature,  are  more 
frequent  with  winds  from  some  directions  than  others.  This  is  shown 
graphically  by  a  device  called  a  "  wind  rose." 

From  an  initial  point,  distances  are  laid  off  in  the  various  directions, 
forty-five  degrees  apart,  proportional  to  the  depth  of  rainfall  occurring 
when  the  winds  are  from  those  particular  directions.  The  ends  of  the 
lines  are  joined  and  the  enclosure  shaded.  Temperature,  vapour  pres- 
sure, etc.,  as  dependent  on  wind  direction,  are  sometimes  represented  in 
this  way  also.  Wind  roses  are,  however,  of  very  little  value,  except  to 
show,  graphically,  the  numbers  from  which  they  are  derived.  The  same 
direction  of  wind,  according  as  it  is  blowing  out  of  an  area  of  high 
barometric  pressure  or  into  an  area  of  low  pressure,  blows  from  very 
different  regions.  Without  separation  of  wind  directions  according  to 


Il8  METEOROLOGY. 

types  of  pressure,  they  are  not  useful  in  the  investigation  of  weather. 
Wind  roses,  made  up  for  places  without  reference  to  types  of  pressure, 
have  been  the  cause  of  long  prevalent  erroneous  conceptions  as  to  the 
way  in  which  rain  originates. 

Thunderstorms.  —  Downpours  of  rain  are  sometimes  accompanied 
by  thunder  and  lightning.  Lightning  is  a  disruptive  discharge  of 
electricity.  The  flash  lasts  but  the  millionth  of  a  second.  Thunder 
is  caused  by  a  sudden  expansion  of  the  air  due  to  the  heating  effect  of 
the  electrical  discharge,  and  the  sudden  rush  of  air  to  the  track  where 
the  rarefaction  is  produced.  The  distance  through  which  lightning 
may  strike  varies  from  a  hundred  or  two  hundred  feet  to  a  mile  or 
more.  Thunder  is  rarely  heard  at  a  greater  distance  from  its  origin 
than  12  miles.  Sound  travels  at  the  rate  of  1118  feet  per  second,  in 
air  at  a  temperature  of  61°,  and  about  one  foot  less  for  every  degree  of 
temperature  below  that.  By  noting  the  time  between  a  lightning  flash 
and  thunder,  the  distance  of  the  origin  can  be  ascertained.  The  light- 
ning flash  and  its  visibility  are  practically  instantaneous.  The  drops 
of  rain,  and  especially  flakes  of  snow,  are  sometimes  so  highly  electrified 
as  to  be  feebly  luminous. 

Rolling  of  Thunder.  —  Thunder  sometimes  rolls  ;  that  is,  the  sound  con- 
tinues for  several  seconds.  This  is  due  in  part  to  the  successive  propaga- 
tion of  the  sound  from  the  different  points  in  the  path  of  the  lightning, 
and  in  part  to  aerial  echo,  just  as  a  wall  reflects  sound.  The  surfaces 
between  layers  of  air,  though  not  solid,  reflect  sound  slightly ;  the  com- 
posite of  the  successive  innumerable  reflections  from  every  vertical 
section  back  of  the  lightning  for  some  distance  makes  up  the  aerial  echo, 
which  is  the  roll  of  the  thunder  finally  dying  away  as  a  faint  rever- 
beration from  a  great  distance. 

Air  Pressure  during  Thunderstorm.  —  There  is  very  little  change  in 
the  air  pressure  before  or  during  thunderstorms.  There  is  a  slight 
fall  in  pressure,  about  o.io  of  an  inch,  in  the  hour  preceding  the  storm 
until  rain  begins,  when  the  pressure  suddenly  increases  0.06  of  an 
inch. 

Description  of  Thunderstorms.  —  An  hour  or  more  before  a  thunder- 
storm, heavy  black  clouds  are  seen  in  the  western  sky.  The  wind  is 
usually  light  from  the  south-west  and  the  temperature  very  high,  85°  to 


WINDS,    THUNDERSTORMS,   AND    TORNADOES.  119 

90°,  and  the  air  very  moist.  As  the  storm  approaches,  the  temperature 
falls  slightly.  A  curtain  of  light  greyish  cloud  is  seen  extending  to  a 
great  height,  while  all  below  is  of  a  uniform  black  tint.  This  front 
presents  a  vista,  very  long  from  south-west  to  north-east  as  compared 
with  its  height.  It  gives  the  appearance  sometimes  known  at  sea  as 
the  " arched  squall."  The  clouds  reach  overhead  a  few  minutes  before 
thunder  is  heard,  and  from  10  to  30  minutes  before  rain  begins. 

Five  minutes  before  the  rain,  the  wind  changes  to  the  north-west, 
and  becomes  very  strong.  The  wind  advances  with  a  rolling  motion 
and  raises  great  clouds  of  dust  in  city  streets.  There  is  usually  a  fall 
of  temperature  of  12  degrees ;  in  the  case  of  hail  it  is  twice  as  great. 

Squall. — At  sea  this  wind  is  called  the  "squall."  These  puffs  are 
sometimes  called  "  white  squalls,"  from  the  whitish  appearance  of  the 
wave-crests  on  the  ruffled  sea.  With  the  "squall"  the  temperature 
falls  rapidly.  The  heaviest  rain  is  at  the  beginning  of  the  squall.  The 
wind  dies  down  as  soon  as  the  rain  begins.  The  thunder  grows  nearer, 
until  the  time  between  flash  and  crash  is  hardly  perceptible.  The 
loudest  thunder  occurs  from  10  to  30  minutes  after  the  beginning  of 
rain.  Lightning  strokes  occur  mostly  during  this  time.  The  relative 
humidity  falls  to  40  per  cent  in  front  of  the  storm,  and  rises  to  80 
inside  of  it.  When  hail  falls,  it  is  in  strips  parallel  to  the  line  of  prog- 
ress of  the  storm.  The  rain  lasts  about  half  an  hour.  From  an  hour 
to  one  hour  and  a  half  after  the  beginning  of  storm  the  rear  edge  is 
overhead,  and  rainbows  appear.  The  last  thunder  is  heard  from  one 
to  two  hours  after  the  beginning  of  storm.  The  rainfall  in  different 
storms  varies  from  half  an  inch  or  less  to  one  inch  and  a  half  or  more, 
usually  no  more  than  half  an  inch.  The  rain  is  greater  in  intensity  the 
shorter  its  duration. 

Isobronts.  —  Thunderstorms  occur  successively  over  strips  of  coun- 
try varying  at  different  times  from  10  to  50  miles  in  width  and  300 
to  400  in  length.  The  motion  of  storm  front  is  from  south-west  to 
north-east  at  the  rate  of  about  34  miles  an  hour  in  summer.  The 
lines  joining  the  points  where  the  first  thunder  is  heard  at  the  same 
instant  of  time  are  called  "isobronts,"  or  lines  of  equal  front. 

Sometimes  the  isobront  is  taken  as  the  mean  of  the  time  when 
first  and  last  thunder  is  heard  at  a  place.  This  is  the  custom  in  France. 


120  METEOROLOGY. 

In  winter  the  front  moves  faster  than  in  summer,  on  the  average  at 
the  rate  of  50  miles  an  hour.  At  some  places  the  storm  is  more  violent 
than  others.  Sometimes  a  storm  will  die  down  during  the  night  and 
begin  again  next  day  where  it  left  off,  and  continue  its  progress.  The 
isobronts  or  storm  fronts  move  in  the  general  direction  of  the  prev- 
alent current  of  the  lower  air.  Sometimes  thunderstorms  progress 
from  a  centre  and  the  isobronts  are  widening  circles.  This  occurs 
in  Switzerland.  In  Italy,  thunderstorms  often  follow  along  in  the 
same  region  at  intervals  of  3  hours  or  multiples  of  three  hours ;  24 
hours  or  the  day  is  a  very  common  interval. 

Lightning.  —  Lightning  follows  an  irregular  zigzag  path  through  the 
air.  What  is  called  sheet  lightning  is  the  reflection  of  distant  zigzag 
lightning  from  the  clouds  and  the  sky  above  the  clouds.  This  is  some- 
times seen  when  a  thunderstorm  is  as  much  as  250  miles  distant. 

Ball  Lightning.  —  Ball  lightning,  seen  at  times,  is  a  slowly  moving  ball 
of  fire  which  finally  explodes.  It  occurs  when  not  only  the  potential  of 
the  electricity  concerned  in  the  production  of  lightning  is  great,  but  when 
the  quantity  of  electricity  is  also  great.  Ball  lightning  can  be  produced 
experimentally  on  a  small  scale  by  sending  the  current  from  a  dynamo 
through  water  contained  in  a  glass.  It  occurs  during  a  thunderstorm, 
mostly  in  wet  places,  along  the  ground,  and  only  at  times  when  there  are 
great  downpours  of  rain.  It  consists  of  a  globe  of  incandescent  rarefied 
air  and  gas  from  the  decomposition  of  vapour  of  water.  The  ruddy  hue 
is  due  to  the  hydrogen,  this  being  characteristic  of  electrical  discharges 
through  hydrogen.  The  least  current  of  air  changes  the  spherical  form. 

Fulgurites.  —  Isolated  trees,  and  trees  on  the  edge  of  a  forest,  are 
more  apt  to  be  struck  by  lightning  than  those  within  it.  Sometimes 
the  lightning  strikes  into  the  earth.  When  this  occurs  in  sand  the  sand 
is  melted  by  the  discharge,  and  the  path  is  marked  by  a  tube  of  vitrified 
sand.  This  is  called  a  "fulgurite."  These  tubes  are  sometimes  30  feet 
long  and  very  irregular,  with  walls  an  inch  thick ;  the  outside  diameter 
of  the  tube  is  about  3  inches.  The  inside  of  the  tube  is  smooth  and 
bright.  Fulgurites  are  very  fragile,  and  can  only  be  taken  out  of  the 
ground  in  short  lengths  of  a  few  inches. 

Frequency  of  Thunderstorms.  —  Thunderstorms  are  of  daily  occur- 
rence in  some  parts  of  the  belt  of  calms  near  the  equator.  The  frequency 


WINDS,    THUNDERSTORMS,   AND    TORNADOES.  121 

diminishes  in  going  north.  From  40  years'  observations  the  average 
number  in  France  in  a  year  is  about  29,  counting  at  a  place  any  day 
when  thunder  is  heard  as  a  day  with  thunderstorm.  In  Iceland  there 
is,  on  the  average,  only  i  a  year ;  in  Finland,  2  ;  in  Java,  97.  In  the 
rainless  area  of  Peru  no  thunder  is  ever  heard.  At  Rio  Janeiro,  Brazil,  a 
thunderstorm  occurs  regularly  at  eleven  o'clock  in  the  morning  in  summer, 
whence  the  local  proverb,  "as  sure  as  a  January  thunderstorm."  The 
same  thing  occurs  at  Puebla,  Mexico,  where,  in  the  summer,  there  is  a 
thunderstorm  every  afternoon  from  two  to  three  o'clock.  These  storms 
are  due  to  the  mountains  in  the  vicinity  producing  rain  from  the  ascend- 
ing currents  along  their  sides. 

Twenty-six  Day  Period. — There  is  some  indication  of  a  twenty-six 
day  period  in  the  frequency  of  thunderstorms.  This  corresponds  to  the 
time  of  rotation  of  the  sun  on  its  axis. 

In  middle  latitudes  thunderstorms  are  most  frequent  in  summer  and 
occur  only  rarely  in  winter.  In  Iceland  they  occur  only  in  winter.  At 
Spitzbergen,  in  latitude  78°,  they  occur  rarely,  and  only  in  summer.  In 
the  United  States  the  average  number  yearly  in  the  lower  Mississippi 
valley  and  Florida  is  50 ;  in  the  region  of  the  Great  Lakes,  20 ;  in  New 
England,  10.  West  of  the  Rocky  Mountains  the  average  number  is  less 
than  10 ;  in  southern  California  a  whole  year  often  passes  without  any. 

Thunderstorms  over  Land  and  Sea. —  Over  the  land  the  time  of  most 
frequent  occurrence  of  thunderstorms  is  in  the  day,  and  from  two  to  five 
o'clock  in  the  afternoon  ;  they  occur,  however,  at  all  hours  of  the  day 
and  night.  Over  the  ocean  thunderstorms  are  essentially  occurrences 
of  the  night. 

Mountainous  and  swampy  regions  are  favourable  to  the  production  of 
thunderstorms. 

Daily  Period.  — There  are  two  maximum  periods  in  the  daily  frequency 
of  thunderstorms  :  one  in  the  afternoon,  and  the  other  not  so  decided  in 
the  early  hours  of  morning  before  sunrise.  The  one  is  clearly  connected 
with  the  warming  of  the  lower  air  by  the  sun,  and  the  other  with  the 
cooling  of  the  upper  strata  by  radiation  in  the  night.  The  thunder- 
storms over  the  ocean  are  principally  due  to  the  latter  cause.  The  time 
of  maximum  frequency  of  lightning  without  thunder  is  at  an  earlier  hour 
than  with  it. 


122  METEOROLOGY. 

Atmospheric  Electricity.  —  The  air  is  usually  in  an  electrified  condi- 
tion, as  shown  by  observations  with  an  electrometer  connected  with  a 
vessel  situated  at  a  height  in  the  air  from  which  water  is  dropping,  which 
enables  it  to  take  the  electricity  from  the  air.  The  potential  of  the  air 
at  Washington,  28  feet  above  the  ground  and  5  feet  from  a  building,  is, 
on  the  average,  55  volts,  being  48  in  summer  and  65  in  winter.  The 
distribution  of  electricity  in  the  air  seems  to  be  due  to  the  varying 
proportion  of  vapour  contained  at  different  times.  When  it  rains,  the 
potential  of  the  air  becomes  the  same  as  the  ground. 

This  may  be  sometimes  due  to  a  film  of  moisture  on  the  insulating 
supports  of  the  electrometer  or  water-dropper,  causing  an  unavoidable 
error  of  observation.  At  times,  in  the  dry  air  of  the  western  part  of 
the  United  States  at  high  altitudes,  light  shocks  are  felt  on  touching 
metal  objects ;  a  tingling  sensation  in  the  fingers  and  ears  is  produced 
by  light  discharges  of  electricity  from  the  air  to  the  ground  through  a 
person's  body. 

Electrical  Storms. — During  such  times  electrical  storms  are  said  to 
prevail.  l^hen  these  occur,  there  are  frequently  thunderstorms  in  the 
vicinity.  On  Pike's  Peak  during  such  storms,  the  telegraph  wires  in 
the  air  appear  luminous  ;  the  revolving  anemometer  cups  resemble  balls 
of  fire  ;  lightning-rods,  trees,  short  stems,  and  other  pointed  objects 
have  brushes  of  light.  Snowflakes  falling  through  the  air  and  striking 
a  horse's  back  give  sparks  of  light.  The  sensations  experienced  during 
such  a  storm  are  decidedly  unpleasant  and  often  even  dangerous. 

Cause  of  Lightning.  — When  an  object  is  charged  with  electricity  and 
there  is  a  flow  of  electricity  from  one  part  of  the  object  to  another,  there 
is  a  difference  of  electric  potential.  The  practical  unit  of  electric  poten- 
tial is  the  volt.  The  wire  connecting  the  two  poles  of  a  single  cell  of 
gravity  battery,  zinc  and  copper  in  a  solution  of  sulphate  of  copper,  is  a 
charged  body  with  a  current  flowing  along  it ;  the  difference  in  potential 
of  the  ends  of  the  wire  is  about  nine-tenths  of  a  volt,  as  long  as  the 
wires  are  not  connected  up ;  it  diminishes  somewhat  on  connecting  the 
wires. 

Substances  vary  very  greatly  in  their  power  of  carrying  electricity 
when  there  is  a  difference  of  potential.  Silver,  copper,  and  all  metals 
are  good  conductors.  Glass,  gutta-percha,  india-rubber,  feathers,  wool, 


WINDS,    THUNDERSTORMS,   AND    TORNADOES.  12$ 

and  air  are  bad  conductors,  or  insulators,  and  carry  very  little  elec- 
tricity. 

If  a  glass  plate  with  a  sheet  of  tin  foil  in  the  centre  on  each  side  be 
connected  one  with  the  positive  and  the  other  with  the  negative  pole  of 
a  cell  of  active  battery,  the  tin  foil  will  take  on  charges  of  electricity, 
and  the  difference  in  their  potential  will  be  that  of  the  poles  of  the 
battery.  There  will  be  no  flow  of  electricity  along  the  wire  after  the 
minute  quantity  is  supplied  which  is  sufficient  to  charge  the  tin  foil. 
This  arrangement  of  glass  and  tin  foil  is  called  a  condenser,  and  is  the 
Leyden  jar  in  principle. 

The  air  is  always  in  an  electric  condition.  There  is  a  difference  of 
potential  between  the  earth  and  the  air  that  increases  with  height  in  the 
air.  The  earth,  the  air  near  the  earth's  surface,  and  the  rarefied  air  at  a 
great  height  has  been  likened  to  a  condenser.  The  earth  is  a  good  con- 
ductor, and  corresponds  to  the  tin  foil  on  one  side  of  the  glass.  The 
lower  layer  of  the  air  for  a  few  miles  above  the  earth's  surface  is  a  bad 
conductor  or  insulator,  and  corresponds  to  the  glass  or  dielectric  of  the 
condenser.  The  upper  highly  rarefied  layer  of  air  is  a  tolerably  good 
conductor  and  corresponds  to  the  other  layer  of  tin  foil  on  the  condenser. 
The  perfect  vacuum  above  the  highly  rarefied  air  is  again  a  non-conduc- 
tor. As  to  the  way  in  which  a  charge  might  originate  on  the  earth  and 
upper  air  considered  in  this  way  as  a  condenser,  it  is  supposed  it  might 
be  due  to  the  rotation  of  the  earth.  The  earth  is  a  magnet,  and  the 
rotation  of  a  magnetic  field  produces  a  current  of  electricity.  A  charged 
condenser  rotating  will  produce  a  magnetic  field,  as  shown  by  experiment. 
It  is  merely  a  surmise  that  the  earth  and  the  upper  air  is  charged  as  a 
condenser  as  indicated.  But  it  is  unquestionably  a  fact  that  the  po- 
tential of  the  air  increases  with  the  height. 

The  energy  in  a  charged  condenser  is  in  the  form  of  a  strain  of  some 
sort  in  the  dielectric  between  the  two  conducting  surfaces.  The  surfaces 
of  the  charged  condenser  have  opposite  kinds  of  electricity,  which  attract 
each  other.  When  the  strain  is  greater  than  the  dielectric  can  stand, 
it  breaks  and  there  is  a  sudden  rush  of  the  electricity  to  equalize  the 
potential,  accompanied  by  a  snapping  sound  and  a  spark.  The  motion 
of  the  electricity  is  not  simply  one  way  but  a  very  great  number  of 
motions,  back  and  forth  several  thousand  times  in  a  small  fraction  of  a 


124  METEOROLOGY. 

second.  The  fact  that  the  spark  is  not  single  but  a  series  of  sparks 
is  shown  by  examining  its  reflection  in  a  rapidly  rotating  mirror,  when 
it  is  extended  out  as  a  series  of  points  of  light  instead  of  a  single  one. 

The  flash  of  lightning  is  an  equalization  of  potential  between  the 
upper  and  lower  layers  of  the  air,  or  the  air  and  the  ground.  Disruption 
of  the  air  and  discharge  occurs  when  the  strain  is  greater  than  0.5  of  a 
gramme  weight  for  every  square  centimetre  of  cloud  surface. 

In  thunderstorms,  the  particles  of  water,  as  they  form  from  vapour, 
take  the  potential  of  the  air  at  the  place  of  formation.  As  the  particles 
coalesce  in  the  contacts  arising  from  continual  intermixture,  the  electric 
potential  on  the  drop  increases.  The  quantity  of  electricity  on  a  body 
is  equal  to  its  electrical  capacity  multiplied  by  its  potential.  The  elec- 
trical capacities  of  spheres  are  proportional  to  their  radii.  Two  spheres 
having  the  same  electrical  potential,  but  the  radius  of  one  being  twice 
that  of  the  other,  the  amount  of  electricity  on  the  larger  one  will  be 
twice  that  on  the  smaller  one.  If  a  thousand  spheres  or  particles  of 
water,  of  the  same  size  and  having  the  same  electrical  potential,  coalesce 
to  form  a  drop,  the  quantity  of  electricity  on  the  drop  is  the  same  as  the 
sum  of  all  on  the  thousand  particles,  but  the  potential  is  100  times  as 
great  as  on  any  one  of  the  particles  separately;  for  the  electrical 
capacity  of  the  drop  is  10,  while  that  of  the  spheres  separately  which 
compose  it  is  1000. 

Striking  Distance.  —  The  distance  through  which  the  disruptive  dis- 
charge takes  place  depends  on  the  electrical  potential ;  the  higher  the 
potential  the  greater  the  striking  distance.  When,  by  the  coalescence 
of  a  large  number  of  particles  of  water,  the  potential  of  the  drops 
throughout  a  cloud  is  increased  enough  to  overcome  the  intervening 
striking  distance  between  the  drops,  the  whole  body  of  raindrops  form- 
ing a  cloud  becomes  a  single  drop,  as  far  as  the  distribution  of  electricity 
is  concerned,  and  its  potential  is  correspondingly  greatly  increased. 
When  the  drops  begin  to  fall,  there  is  a  disruptive  discharge  or  flash 
of  lightning  as  soon  as  the  rain  front  reaches  the  striking  distance  from 
the  earth.  A  number  of  seconds  after  a  heavy  discharge  of  lightning 
there  is  often  noticeable  a  marked  increase  in  the  intensity  of  the  down- 
pour of  rain. 

The  chance  of  a  tower  or  steeple  being  struck  by  lightning  is  forty 


WINDS,    THUNDERSTORMS,   AND    TORNADOES. 


125 


times  as  great  as  for  an  ordinary  building.  A  tree  in  a  clearing  or  on 
the  edge  of  a  wood  is  more  likely  to  be  struck  that  one  in  the  depth  of 
a  forest. '  Oak  is  more  likely  to  be  struck  than  any  other  variety  of  tree. 
On  the  average,  about  four  persons  in  a  million  are  killed  in  a  year  in 
France  and  Germany,  by  lightning.  Two  hundred  persons  annually  were 
killed  in  the  United  States  by  lightning,  from  1880  to  1890.  About 
eight  hundred  fires  in  a  year  are  caused  by  lightning,  in  the  United 
States.  It  is  a  prevalent  misapprehension  that  lightning  never  strikes 
twice  in  the  same  place,  or  only  strikes  isolated  objects. 

Cause  of  Thunderstorms.  —  Thunderstorms  are  due  to  a  rapid 
decrease  of  temperature,  with  ascent  in  the  air  and  the  presence  of  a 
great  deal  of  moisture  in  the  lower  air.  The  veil  of  cirrus  clouds  seen 


--IS1 


^20°-  -5 


FiG.    20. 

just  before  a  thunderstorm  is  observed  to  be  much  lower  at  such  times 
than  ordinarily.  Observations  by  aeronauts,  made  in  balloon  ascents 
at  times  of  thunderstorms,  show  a  rapid  decrease  of  temperature  with 
height.  When  the  decrease  of  temperature  with  height  is  greater 
than  0.58  of  a  degree  in  one  hundred  feet  of  ascent,  the  equilibrium  of 
the  layers  of  air  is  disturbed,  and  the  lower,  warm  air  begins  to  change 
places  with  the  upper,  colder  air.  This  tendency  of  the  air  to  rise  is 
increased  by  the  moisture  of  the  lower  air  when  present  in  consider- 
able quantities,  being  only  two-thirds  of  the  density  of  air  of  the  same 
pressure.  When  the  rate  of  decrease  is  not  much  greater  than  0.58 
of  a  degree,  or  extends  only  throughout  a  layer  of  air  three  or  four 
hundred  feet  thick,  the  interchange  of  air  above  and  below  goes  on 
slowly  and  imperceptibly.  But  when  the  rate  is  exceeded  very  much, 
and  extends  throughout  a  great  depth  of  air,  sufficient  to  carry  a 
large  body  of  the  surface  air  to  a  point  where  the  temperature  is  a 


126  METEOROLOGY. 

good  deal  below  the  dew-point,  then  the  conditions  exist  favourable 
to  a  thunderstorm.  The  rain  occurs  first  in  the  centre  of  the  ascend- 
ing current.  This  cools  the  lower  air  in  its  descent,  coming  from  a 
colder  region  above.  It  also  cools  the  air  some  by  evaporation.  The 
result  of  this  is  that  the  air  in  the  centre,  becoming  denser  than  the 
surrounding  air,  begins  to  descend.  This  is  shown  in  Fig.  20,  which  is 
a  longitudinal  section  of  a  thunder  cloud  in  motion  from  left  to  right. 
The  scale  on  right  is  the  height  in  hundreds  of  meters.  The  highest 
clouds  are  e,  e,  about  a  mile  high.  The  currents  are  shown  by  the  direc- 
tion of  the  arrows  below  the  thunder  cloud,  a,  a.  The  motion  of  the  air 
is  a  whirl  around  a  horizontal  axis.  Over  the  stretch  of  country  shown, 
when  the  rain  begins  to  fall,  there  is  a  downward  and  outward  current 
at  b.  At  this  point  the  barometric  pressure  is  greater  than  round  about 
it.  There  is  also  a  great  contrast  of  temperature  in  the  front.  These 
are  the  conditions  that  precede  the  thunderstorm.  The  burst  of  wind  or 
the  squall,  the  change  in  wind  from  south-west  to  north-west,  is  the  pas- 
sage over  a  place  of  this  downward  cold  current.  The  sudden  increase  of 
pressure  at  the  commencement  of  rain  is  largely  due  to  the  column  of 
cold  air  being  of  greater  weight  than  the  equally  high  column  of  warm 
air  just  preceding  it. 

Increase  of  Pressure. — The  pressure  increase  may  be  due  to  some 
extent  to  the  forcing  down  of  the  air  caused  by  the  falling  drops  of 
water.  Some  part  of  it,  too,  may  be  due  to  the  increase  of  pressure 
due  to  evaporation  from  the  water-drops.  The  difference  in  tempera- 
ture of  the  air  is  adequate  to  account  for  the  increase  of  pressure.  A 
column  of  air  2  degrees  colder  on  the  average  than  the  surrounding 
air,  and  of  a  height  such  as  is  ordinarily  active  in  thunderstorms,  would 
produce  a  rise  of  0.07  of  an  inch  in  pressure.  The  difference  in  tem- 
perature at  the  surface  of  the  earth  extends  up  through  only  a  relatively 
small  height. 

Cause  of  Squall.  —  A  difference  of  0.07  of  an  inch  in  pressure  in  the 
distance  ordinarily  observed  is  capable  of  producing  a  wind  of  43  miles 
an  hour,  which  accounts  amply  in  all  cases  for  the  squall. 

Tornado. — A  tornado  is  a  specially  violent  form  of  thunderstorm.  A 
characteristic  feature  is  a  funnel-shaped  cloud  dipping  down  from  the 
main  storm-cloud,  at  times  reaching  the  surface  of  the  earth  and  caus- 


WINDS,    THUNDERSTORMS,   AND    TORNADOES. 


127 


ing  devastation  wherever  it  touches.  The  very  great  velocity  of  the 
wind  in  the  vicinity  of  the  funnel,  estimated  to  be  300  miles  or  more 
an  hour,  destroys  everything  in  the  path  of  the  funnel  cloud,  uprooting 
trees,  levelling  houses  to  the  ground,  and  at  times  blowing  locomotives 
off  the  track  and  lifting  iron  bridges  from  their  foundations.  The  fun- 
nel cloud  advances  over  the  earth  from  south-west  to  north-east  at  the 
rate  of  about  30  miles  an  hour.  It  bounds  along,  sometimes  skipping 
above  the  earth  and  then  coming  down  again  and  swaying  from  side  to 
side  as  it  renews  its  ravages.  The  swath  of  destruction  corresponds 
to  the  width  of  the  funnel,  and  varies  in  different  cases  from  50  feet  to 
1000  feet,  and  its  path  extends  over  a  strip  of  country  from  5  to  50 
miles  in  length. 

A  characteristic  tornado  cloud  and  funnel  is  shown  in  Fig.  21, 
obtained  from  a  photograph  of  the  cloud  seen  at  Howard  Mines  Com- 
pany, South  Dakota,  August  28,  1884,  as  it  passed  22  miles  to  the  west 


FIG.  21. 


in  a  south-easterly  direction.  A  peculiar  form  of  cloud  invariably  pres- 
ent from  30  minutes  to  2  hours  before  the  tornado  is  characterized  by 
ball-like  masses  of  light  grey  or  white  colour  often  in  long  lines  on  a 
darker  background  or  in  clumps,  often  three  or  more  side  by  side,  the 
under  side  circular,  the  upper  indefinite  or  shading  into  the  main  cloud. 
At  times  this  gives  the  appearance  of  having  scallops,  or  a  shell-like 


128  METEOROLOGY. 

edge,  when  the  tufts  are  at  the  lower  margin  of  the  cloud ;  the  under 
side  is  perfectly  round,  the  upper  side  streaming  out  like  cotton  balls 
partially  unwound. 

Noise  of  Tornado.  —  A  tremendous  roaring  sound  accompanies  a 
tornado  in  its  progress,  similar  to  the  rushing  of  a  thousand  express- 
trains. 

Tornado  Wind. — The  wind  blows  spirally  inward  around  the  funnel, 
contrary  to  the  direction  of  motion  of  the  hands  of  a  watch,  or  from  the 
east  around  by  the  north,  the  west,  and  the  south.  The  funnel  marks 
the  surface  where  the  moisture  in  the  air  in  its  ascent  is  condensing. 
It  is  a  region  of  very  low  pressure,  produced  by  the  centrifugal  ten- 
dency of  the  air  from  the  centre  of  rotation.  From  the  size  and  weight 
of  articles  carried  up,  it  is  estimated  that  the  upward  velocity  of  the  air 
must  be  at  least  at  the  rate  of  176  miles  an  hour.  Iron  objects  weigh- 
ing 1500  pounds  have  been  moved  as  much  as  20  feet;  objects  weigh- 
ing 100  pounds  have  been  carried  several  hundred  feet ;  pieces  of  tin- 
roofing  have  been  carried  17  miles  ;  a  letter  has  been  carried  45  miles. 
The  upward  motion  is  in  a  zone  some  distance  from  the  centre.  There 
probably  is  no  upward  motion  of  the  air  right  at  the  centre  or  axis. 
Fowls  have  been  stripped  of  feathers  in  tornadoes  and  remained  other- 
wise uninjured.  All  attempts  to  produce  this  result  by  the  sudden 
exhaustion  of  air  about  fowls  in  closed  vessels  have  failed.  When  fired 
from  a  cannon,  the  feathers  are  plucked,  but  the  body  is  torn  to  pieces 
when  the  velocity  is  340  miles  per  hour.  The  wind  velocity  in  a  tor- 
nado must  be  less  in  some  instances  than  this,  and  probably  a  good 
deal  less.  By  means  of  a  mechanical  air-blast,  straws  have  been  driven 
into  wood  a  depth  of  one-tenth  of  an  inch  with  a  velocity  of  the  air  of 
135  to  1 60  miles  an  hour.  A  velocity  at  least  as  high  as  that  must  have 
occurred  in  the  tornado  at  Washington,  Ohio. 

Tornado  Rain.  —  Rain  invariably  precedes  a  tornado  cloud  from  10  to 
30  minutes,  and  hail  follows  it.  The  rapid  ascent  of  nearly  saturated 
air  gives  rise  to  great  downpours  of  rain  in  tornadoes.  The  hail  that 
accompanies  a  tornado  is  usually  of  very  great  size.  Generally  there  is 
a  great  deal  of  lightning  accompanying  a  tornado. 

Tornado  Lightning. — The  funnel  often  has  a  ruddy  smoky  hue,  as  if 
the  electrical  discharges  were  continuous. 


WINDS,    THUNDERSTORMS,    AND    TORNADOES.  1 29 

Time  of  Tornado  Occurrence.  —  Tornadoes  almost  invariably  begin  in 
the  afternoon  from  two  to  five  o'clock.  There  is  a  previous  great 
accumulation  of  cloud-mass  going  on  for  several  hours  before  a  tornado 
begins.  This  slow  accumulation  of  energy  is  a  distinctive  feature  of 
the  tornado  cloud. 

Moisture  and  Tornadoes.  —  Tornadoes  require  for  their  production  a 
great  deal  of  moisture  in  the  air  and  diminishing  temperature  extending 
to  a  great  height.  That  the  ascending  current  must  reach  to  a  great 
height  appears  from  the  universal  occurrence  of  hail.  The  air  must  go 
high  enough  to  freeze  the  water ;  and  to  do  this  readily  the  temperature 
must  be  considerably  below  freezing-point.  As  latent  heat  is  given  off 
by  the  condensation  of  vapour,  it  is  a  warming  process,  and  there  must 
be  abundant  masses  of  cold  air  to  dispose  of  this  heat  by  mixture  or  by 
cold  of  expansion  in  ascending. 

Vertical  Ring. — That  there  is  a  ring  of  vertically  revolving  air  around 
an  up-rushing  current  is  shown  by  the  concentric  layers  of  soft  and  hard 
ice  in  hailstones.  The  hailstones  take  on  a  coating  of  snow  in  the 
higher  air,  but  when  in  the  course  of  its  path  it  descends  into  the  lower 
air  this  snow  is  melted  and  compacted  somewhat ;  in  ascending  again 
the  mass  takes  on  another  coating,  and  so  on.  The  upward  velocity 
required  to  sustain  such  masses  is  not  very  great.  A  vertical  velocity 
of  only  17  feet  a  second  is  sufficient  to  carry  up  raindrops  one-tenth  of 
an  inch  in  diameter. 

Spiral  Motion.  —  When  the  upward  velocity  of  the  air  is  great  the 
ascending  air  tends  to  a  spirally  outward  and  upward  motion,  similar  to 
the  spirally  inverted  and  downward  motion  of  water  running  through  a 
hole  in  a  basin  or  bath  tub. 

Lowering  of  Pressure.  —  The  rotary  motion  of  the  air  tending  to  fly 
off  at  a  tangent  develops  a  centrifugal  force  which  produces  a  lowering 
of  pressure  in  the  whirl.  The  bursting  of  houses  by  the  tornado  funnel 
passing  over  them,  and  the  arching  of  floors  which  occur  at  times,  show 
that  there  is  a  very  great  reduction  of  pressure  near  the  centre  of  the 
whirl.  In  the  milder  forms  of  tornado  this  is  marked  by  the  sudden 
purring  of  volumes  of  soot  from  chimneys,  showing  a  sudden  up-rush  of 
air  on  the  release  of  the  outside  air  pressure. 

Motion  of  Tornado.  —  The  fact  that  the  motion  of  rotation  in  a  .tor- 


130  METEOROLOGY. 

nado  is  contrary  to  the  motion  of  the  hands  of  a  watch,  and  that  the 
tornado  moves  from  south-west  to  north-east,  has  never  been  satisfacto- 
rily explained.  The  two  characteristics  are  so  marked  and  invariable 
that  they  are  probably  related  in  some  way  to  the  rotation  of  the  earth 
and  the  influence  of  its  deflecting  force  on  air  currents. 

Place  of  Tornado  Occurrence. — Tornadoes  occur  most  frequently  in 
the  south-east  quarter  of  an  area  of  low  barometric  pressure.  The 
condition  most  favourable  to  their  occurrence  is  the  case  of  a  warm 
current  of  air  underrunning  a  cold  one.  The  south-east  quarter  of  a 
low  area  has  more  winds  with  northerly  and  southerly  components  than 
any  of  the  other  quarters. 

Tornadoes  in  the  United  States.  —  Tornadoes  are  of  frequent  occur- 
rence in  the  United  States,  and  cause  every  year  loss  of  life  and  destruc- 
tion of  property.  The  month  of  most  frequent  occurrence  is  May ;  next 
April,  then  June  and  July.  They  seldom  occur  in  winter.  They  occur 
everywhere  in  the  country  east  of  the  hundredth  meridian,  but  never  to 
the  west  of  this.  The  regions  of  frequent  occurrence  are  the  upper 
Mississippi  and  lower  Missouri  valleys. 

Series  of  Tornadoes.  —  Tornadoes  occur  in  series  on  the  same  day, 
moving  in  the  same  direction,  their  paths  a  few  miles  apart.  A  series 
of  this  kind,  comprising  60  separate  tornadoes,  occurred  after  ten  o'clock 
on  the  morning  of  February  9,  1884,  in  Illinois,  Kentucky,  Tennessee, 
Virginia,  North  Carolina,  South  Carolina,  Georgia,  and  Mississippi,  in 
which  800  people  were  killed,  2500  wounded,  and  10,000  houses  were 
destroyed. 

About  60  tornadoes  of  great  destructiveness  have  occurred  in  the 
United  States  in  the  20  years  from  1870  to  1890,  in  which  numerous 
lives  were  lost,  and  the  damage  to  property  in  every  case  exceeded 
£  200,000. 

Waterspouts.  —  Waterspouts  on  lakes  and  the  ocean  are  columns  of 
water  the  result  of  a  rotary  motion  in  the  air  of  no  very  great  intensity. 


CHAPTER  VI. 

OPTICAL  APPEARANCES. 

Refraction.  —  Air  is  a  refracting  medium,  like  glass,  water,  and  dia- 
mond. The  rays  of  light  from  an  object  to  the  eye  as  they  pass  through 
air  of  varying  density  along  their  paths  are  not  straight  lines,  but 
slightly  curved.  The  effect  of  refraction  is  to  make  the  heavenly  bodies  — 
the  sun,  moon,  and  stars  —  appear  higher  up  in  the  sky  than  they  really 
are.  The  judgment  of  the  direction  of  an  object  is  based  on  the  direc- 
tion of  the  last  part  of  a  ray  of  light  entering  the  eye,  and  which  is 
tangent  to  the  curved  path  of  the  ray. 

Lifting  of  Sun. — When  the  sun  appears  with  its  lower  edge  just 
touching  the  horizon,  it  is  by  refraction  lifted  up  in  the  sky  more  than 
its  whole  diameter,  which  is  32'  3",  so  that,  though  visible,  it  is  really 
below  the  horizon.  This  fact  was  first  observed  about  the  year  1664, 
on  the  occasion  of  a  Dutch  exploring  party  wintering  in  the  arctic 
regions.  From  the  latitude  of  the  place,  it  was  calculated  that  the  sun 
would  reappear  on  a  certain  date.  In  the  spring  the  sun  was  visible 
three  weeks  before  the  time.  On  the  return  of  the  party  to  Holland 
the  explanation  of  the  fact,  as  due  to  refraction,  was  given  by  Descartes. 

Flattening  of  Sun.  —  The  flattened  appearance  of  the  sun  when  near 
the  horizon  is  also  due  to  refraction,  the  lower  edge  being  lifted  up  more 
than  the  upper  one.  The  apparent  lifting  of  objects  diminishes  rapidly 
with  altitude  in  the  sky.  At  an  altitude  of  30°,  the  sun  is  lifted  only 
2'  40". 

Lifting  of  Objects.  —  Objects  on  the  surface  of  the  earth  also  are 
lifted  up  in  appearance  by  the  refraction  of  the  air.  Distant  objects  on 
the  surface  of  the  earth,  such  as  ships  and  mountains,  are  hidden  from 
view  by  the  curvature  of  the  earth,  unless  they  reach  up  to  a  certain 
height.  This  height  is  about  in  proportion  to  the  square  of  the  distance 


132  METEOROLOGY. 

of  an  object.  Objects  can  be  seen  at  about  one-seventh  greater  distance 
on  account  of  refraction  than  would  be  the  case  without  it.  This  effect 
of  the  air  varies  at  different  times.  In  sighting  over  long  lines  in  the 
surveying  operations  of  triangulation,  when  the  two  distant  points  are 
nearly  but  not  quite  intervisible,  as  can  be  computed  from  the  known 
heights,  the  curvature  of  the  earth,  and  the  average  refraction,  they  will 
become  intervisible  on  occasions  of  extraordinary  refraction.  With 
extraordinary  refraction  the  distant  place  comes  into  view,  and  then 
slowly  sinks  out  of  sight  to  reappear  again.  This  is  repeated  every 
few  minutes,  and  is  a  phenomena  that  is  seen  most  frequently  in  the 
evening. 

Looming  up.  —  Distant  objects  are  often  seen  in  the  evening 
lengthened  out  vertically  by  refraction.  This  is  called  "  looming  up." 

Mirage.  —  Inverted  images  of  distant  trees  and  ships  are  often  seen 
near  the  horizon  in  the  sky  below  the  direct  images.  This  is  called 
"mirage."  This  name  is  also  given  to  the  delusive  appearance  of  water, 
like  a  lake-surface  sometimes  seen  in  the  desert,  which  is  in  reality  the 
refracted  image  of  the  sky  reaching  the  eye  from  such  a  direction  as  to 
make  it  appear  on  the  ground.  The  layers  of  air  at  different  heights 
have  a  varying  density  on  account  of  the  heat  from  the  ground,  such 
that  the  ray  of  light  is  so  curved  in  passing  through  them  that  the  tan- 
gent to  the  ray  at  the  eye  of  the  observer  comes  from  below,  and  there- 
fore the  image  seems  as  if  it  were  below  the  ground. 

In  Australia,  in  recent  years,  the  discovery  of  a  great  lake  in  the 
interior  of  the  country  was  announced,  which  turned  out  to  be  only  an 
observation  of  a  mirage. 

Dispersion. — White  light  is  a  mixture  of  various  coloured  lights. 
When  a  beam  of  light  is  separated  into  its  constituent  colours,  as  in 
a  spectrum,  it  is  called  "  dispersion."  Dispersion  is  produced  by  either 
refraction  or  diffraction  or  interference. 

Rainbows.  —  Rainbows  are  due  to  the  combined  effects  of  refraction 
and  interference  of  the  sun's  rays  as  they  pass  through  drops  of  falling 
rain,  and  are  reflected  from  the  interior  surfaces  of  the  drops.  The 
colours,  counting  from  the  interior  of  the  rainbow  arch  outward,  are  in  the 
order  of  the  spectrum, — violet,  indigo,  blue,  green,  yellow,  orange,  and  red. 
The  purity  of  the  colours  depends  on  the  uniformity  and  size  of  the 


OPTICAL  APPEARANCES.  133 

drops  of  rain.  The  width  of  the  arch  is  2,\  degrees,  varying  some- 
what with  the  dimensions  of  the  drops.  The  radius  from  the  centre 
of  the  arch  to  the  extreme  outside  red  is  42.1  degrees.  The  centre 
of  the  arch  is  directly  opposite  the  sun.  Each  observer  sees  his  own 
rainbow  about  his  own  anti-solar  point. 

Secondary  Bow.  —  A  secondary  rainbow  with  the  violet  above  and 
the  red  below  is  formed  outside  the  first  by  light  twice  reflected  from 
the  interior  of  the  drops.  The  radius  is  8.5  degrees  greater  than  the 
first.  Under  the  first  rainbow,  variegated  green  and  reddish  bands  are 
sometimes  seen.  These  indicate  that  the  drops  producing  the  rainbows 
are  very  small. 

Centre  of  Rainbow.  —  When  the  sun  is  at  a  greater  altitude  than  42.  i 
degrees,  the  top  of  the  rainbow  is  just  at  the  horizon  and  no  rainbow 
can  be  seen.  On  a  high  mountain  or  in  a  balloon,  when  the  sun  is  near 
the  horizon,  the  rainbow  can  be  seen  as  a  complete  circle. 

Wind  Galls.  —  Portions  of  rainbows  seen  at  times  are  called  by 
sailors  "wind  galls." 

Reflections  of  rainbows  are  sometimes  seen  from  a  surface  of  quiet 
water. 

Moonlight  Rainbows.  —  Rainbows  are  sometimes  seen  by  moonlight. 
The  colours  are  very  dim. 

Fog  Bow. — When  the  particles  of  rain  are  less  than  the  o.oi  of  an 
inch  in  diameter,  the  interference  is  more  effective  than  simple  refrac- 
tion, and  the  colours  of  the  rainbow  are  mixed  and  confused  and  the 
arch  widened  out.  It  then  becomes  a  faint  band  of  light  5  degrees 
wide,  with  a  slight  rosy  tint  on  the  outside,  and  is  then  called  a  "  fog 
bow."  The  radius  is  3.5  degrees  less  than  that  of  the  rainbow.  Fog 
bows  are  sometimes  known  as  fog-eaters,  from  the  fact  that  when  they 
appear,  by  the  sun  shining  on  them,  the  fogs  are  quickly  dissipated  by 
the  heat  converting  the  fog  to  vapour. 

Brocken  Spectre.  —  The  shadow  of  a  person  seen  on  a  fog,  usually 
observed  from  a  mountain  top,  is  called  the  "  Brocken  Spectre,"  from 
the  name  of  a  mountain  in  Germany. 

Glories.  —  Coloured  circles  seen  around  the  shadow  of  a  person's  head 
on  a  fog  are  called  "  glories." 

Corona.  —  Small  coloured  rings  or  full  circles  of  blue,  white,  golden, 


134  METEOROLOGY. 

and  red,  3,  6,  and  10  degrees  in  diameter  around  the  sun  or  moon,  but 
seen  principally  around  the  moon,  are  called  "coronas."  Coronas  are  red 
outside.  They  are  due  to  the  diffraction  of  light  in  passing  between 
the  minute  particles  of  cloud  or  haze  covering  the  moon.  Diffraction 
is  the  name  given  to  the  peculiar  action  of  the  edge  of  a  body  in  dis- 
persing light  passing  close  to  it.  The  smaller  the  intervals  between 
the  particles,  provided  the  rays  of  light  are  not  entirely  cut  off,  the 
greater  the  diameter  of  the  rings.  The  same  appearances  of  coloured 
rings  are  seen  on  looking  at  any  source  of  light  through  a  network 
of  fine  meshes  or  a  pane  of  glass  covered  with  moisture  from  the  breath 
or  with  fine  dust. 

Diminishing  diameter  of  corona  shows  that  the  particles  of  fog  are 
coalescing,  and  may  fall  as  rain  when  sufficiently  large. 

Aureole. — The  ring  of  white  light  about  12  degrees  wide  with  the 
outside  border  of  a  ruddy  tint,  sometimes  seen  around  the  sun  or  moon, 
is  called  an  "  aureole."  The  outside  diameter  of  ring  is  22  degrees,  the 
inside  10  degrees,  and  within  this  the  sky  is  dark. 

Halos. — The  large  circles,  one  21.6  degrees  in  diameter,  the  other 
45.8  degrees,  and  occasionally  one  of  90  degrees,  seen  around  the  sun 
and  moon,  sometimes  coloured,  but  more  usually  white,  are  called  "halos." 
Any  ring  greater  than  16  degrees  in  diameter  is  a  halo.  Halos  are  red 
inside. 

Parhelia,  Paraselenae :  Mock  Suns  or  Sun-dogs,  Mock  Moons.  —  Some- 
times supernumerary  circles  not  concentric  with  the  sun  or  moon 
appear.  These  consist  of  intersecting  and  tangent  or  contact  arches. 
Bands  or  stripes  of  light  also  appear  at  times.  The  intersection  of 
these  with  the  halos  produce  spots  of  more  brilliant  illumination  than 
at  other  parts  of  the  halos. 

These  are  called  "mock  suns"  or  "parjielia,"  and  sometimes  "sun- 
dogs."  Similar  appearances  around  the  moon  are  called  "mock  moons" 
or  "paraselenae." 

Parhelic  Circle.  —  Sometimes  a  circle  of  light  extends  all  around  the 
sky  at  the  height  of  the  sun  or  moon  and  parallel  to  the  horizon.  This 
is  called  a  "parhelic  circle." 

Parhelia  are  sometimes  visible  on  the  parhelic  circle  at  a  distance 
of  1 20  degrees  from  the  sun ;  very  rarely  they  are  also  seen  at  distances 
of  50  degrees  and  98  degrees. 


OPTICAL  APPEARANCES.  135 

Anthelion.  —  An  image  of  the  sun  directly  opposite,  or  180  degrees 
distant  from  the  sun,  is  sometimes  called  a  "parhelion,"  but  is  more 
properly  known  as  an  "anthelion." 

Luminous  Cross.  —  Sometimes  •  columns  of  light  are  seen,  when  the 
sun  is  near  the  horizon,  extending  vertically  10  to  15  degrees  above  the 
sun  ;  sometimes  a  similar  column  is  seen  extending  down  from  the  sun. 
When  both  occur  in  connection  with  part  of  a  parhelic  circle,  a  lumi- 
nous cross  is  produced.  These  columns  are  due  to  the  reflection  of 
sunlight  from  the  upper  and  lower  facets  of  ice-crystals  floating  in  the 
air. 

These  various  optical  appearances  are  all  due  to  the  reflection,  refrac- 
tion, diffraction,  and  interference  of  light  in  passing  near  particles  of 
water  and  ice-crystals  in  the  air.  They  are  most  frequently  to  be  seen 
in  the  arctic  regions,  where  there  is  an  abundance  of  ice-crystals  in  the 
air  for  their  formation. 

Blue  Sky.  —  The  blue  colour  of  the  sky  is  due  to  the  sunlight  reflected 
by  minute  globules  of  water  in  the  air.  The  polarization  shows  the  light 
to  be  reflected.  The  greatest  amount  of  polarization  is  in  a  plane  per- 
pendicular to  the  sun's  rays.  This  reflection  of  polarized  light  is  called 
"  selective  reflection  ' ' ;  larger  particles  would  reflect  red  light. 

The  thicker  the  layer  of  air  the  sun's  rays  pass  through  the  more  its 
light  is  absorbed.  When  there  is  a  great  deal  of  dust  and  moisture 
in  the  air  the  sun  at  sunset  assumes  a  ruddy  hue,  and  the  clouds  at 
times  are  red. 

Red  Sunsets. — The  red  sunsets  of  August,  1883,  and  the  red  skies 
which  glowed  long  after  dark,  observed  over  many  parts  of  the  world, 
are  considered  to  have  been  due  to  the  reflection  of  sunlight  by  parti- 
cles of  vapour  and  dust  probably  high  up  in  the  air.  The  vapour  and 
dust  is  supposed  to  have  come  from  the  great  eruption  of  matter  thrown 
into  the  air  from  Krakatoa,  a  volcano  in  the  island  of  Java.  The  ashes 
and  vapour  were  carried  very  high  in  the  air  and  over  all  parts  of  the 
world  by  the  general  circulation  of  the  air.  It  first  extended  rapidly 
eastward  in  the  tropics  and  made  a  circuit  of  the  earth  in  12  or  13 
days,  showing  a  velocity  in  the  upper  eastward  current  of  about  83 
miles  an  hour. 

Twilight.  —  Twilight   is  the  illumination  after  sunset  produced  by 


136  METEOROLOGY. 

the  reflection  of  sunlight  from  the  upper  air.  From  its  duration  it  is 
estimated  there  is  not  much  light  reflected  by  any  air  there  may  be 
above  a  height  of  36  miles. 

In  high  latitudes,  in  summer,  the  twilight  lasts  a  long  time  on 
account  of  the  inclined  direction  to  the  horizon  in  which  the  sun 
descends.  In  latitude  56°,  on  June  21,  a  book  of  ordinary  print  can  be 
read  by  twilight  as  late  as  nine  o'clock  in  the  evening.  Near  the  equator 
and  in  the  tropics,  where  the  sun  goes  down  almost  vertically,  twilight 
lasts  but  twenty  minutes.  A  similar  rapid  darkening  is  sometimes  due 
to  the  extinction  of  light  by  haze  in  the  air,  as  well  as  to  the  rapid 
descent  of  the  sun. 

Light  through  rifts  in  the  clouds  illuminating  dust  particles  is  some- 
times known  as  the  sun's  drawing  water;  the  beams  are  called  by 
sailors  the  sun's  "backstays." 

The  divergent  rays  sometimes  seen  after  sunset  and  before  sunrise 
dividing  the  sky  into  segments  are  known  as  "crepuscular  rays."  In 
Japan,  they  are  known  by  the  name  of  the  ropes  of  Maui. 

Ice-blink.  —  Ice-blink  is  a  peculiar  whitening  of  the  sky,  low  down 
near  the  horizon,  seen  in  the  arctic  regions  on  approaching  an  ice-floe. 
It  looks  brightest  in  clear  weather,  and  is  seen  at  a  distance  of  30  miles 
from  the  ice. 

Snow-banners.  —  Snow-banners  are  long  divergent  beams  or  stream- 
ers from  the  tops  of  mountains  sometimes  seen  from  a  great  distance, 
30  or  40  miles.  They  are  due  to  the  sunlight  on  fine  particles  of  snow 
contained  in  the  air  currents  diverted  upward  by  mountains.  They  are 
only  seen  when  very  strong  winds  prevail. 

AURORA   BOREALIS,    OR   NORTHERN    LIGHTS. 

Arch.  —  The  faint  luminous  arch  seen  at  times  in  the  night  low  down 
in  the  northern  sky  is  the  aurora  borealis,  or  northern  lights.  Below 
the  arch  is  what  is  known  as  the  dark  segment.  The  lower  line  of  the 
arch  is  often  irregular  in  outline,  but  is  always  well  defined,  showing  a 
sharp  separation  between  light  and  darkness. 

Streamers.  —  Slender  spears  of  well-defined,  bright  light,  called 
"  streamers,"  extend  up  into  the  sky  from  the  arch  for  a  distance  of  20 
and  30  and  sometimes  even  90  degrees.  They  are  from  half  a  degree  to 


OPTICAL  APPEARANCES.  137 

three  degrees  wide,  and  move  or  dance  from  side  to  side  to  the  right  and 
left  and  are  of  a  pale  yellow  colour.  Sometimes  they  are  known  as  merry- 
dancers.  The  arch  is  continually  rising  and  falling.  In  high  latitudes 
there  are  generally  several  arches.  The  plane  of  the  arch  is  generally 
perpendicular  to  the  direction  the  magnetic  needle  takes. 

Auroral  Corona. — Sometimes  beams  of  light  shoot  up  all  at  once  from 
every  part  of  the  horizon  and  form  a  tremulous  mass  of  feathery  flame 
in  the  zenith.  This  is  the  corona.  These  beams  are  parallel  to  the 
direction  of  the  dip-needle.  The  apparent  convergence  is  the  effect 
of  perspective. 

Duration  of  Aurora.  —  Complete  auroral  displays  seldom  last  more  than 
one  hour.  Partial  auroras  last  a  whole  night  or  occur  on  two  successive 
nights,  possibly  continuing  during  the  day,  though  invisible.  Sometimes 
the  auroral  light  is  of  a  crimson  hue,  yellow,  green,  or  blue. 

Extent  of  Visibility. — Auroras  are  at  times  seen  simultaneously  over 
a  great  extent  of  the  world,  from  California  to  Russia  and  from  Jamaica 
to  Labrador.  Not  more  than  six  auroras,  however,  are  visible  in  a 
century  as  far  south  as  latitude  20°.  Towards  the  north,  they  increase 
in  frequency,  and  the  arches  are  higher  up  in  the  sky.  On  an  average, 
there  are  visible  in  a  year  at  latitude  40°,  10  auroras ;  at  latitude  42°, 
20 ;  at  latitude  45°,  40 ;  and  at  latitude  50°,  80.  Between  latitude  50° 
and  60°  they  are  visible  almost  every  clear  night.  From  latitude  62° 
towards  the  north  they  diminish  in  frequency.  At  62°  the  number 
visible  in  a  year  is  40  ;  at  67°,  20  ;  and  at  78°,  10. 

Frequency.  —  Auroras  are  less  frequent  in  winter  than  at  other 
seasons. 

The  number  of  auroras  varies  greatly  in  different  years.  Periods  of 
greatest  frequency  are  about  56  years  apart,  as  indicated  by  records 
of  the  whole  world  since  1742.  The  years  of  greatest  frequency  were 
1 787  and  1845.  There  is  some  indication  of  a  minor  period  of  maximum 
frequency  equal  to  about  ten  years. 

There  is  a  belief  in  some  places  that  auroras  are  followed  by  cold 
weather;  an  examination  of  the  records,  however,  does  not  show  this 
to  be,  as  a  rule,  the  case.  Being  visible  only  when  the  sky  is  clear,  the 
temperature  is  usually  lower  when  they  are  seen  than  at  other  times. 

Magnetic  Storms.  —  During  auroral  displays  there  is  a  disturbance  of 


138  METEOROLOGY. 

the  magnetic  needle,  causing  it  in  middle  latitudes  to  swing  in  half  an 
hour  three  or  four  degrees  in  extreme  cases  from  its  average  direction. 
The  horizontal  force  of  the  earth's  magnetism  may  change  in  the  same 
time  by  one-ninth  of  its  whole  amount.  These  disturbances  of  the 
needle  occur  at  very  nearly  the  same  time  at  places  hundreds  of  miles 
apart.  When  the  auroral  light  is  rosy  coloured,  the  magnetic  disturbances 
are  said  to  be  greater  than  when  white.  Such  disturbances  of  the 
needle  are  called  "magnetic  storms."  At  such  times  there  are  also  strong 
electric  currents  in  telegraph  wires  and  cables,  interfering  with  their 
working.  These  may  be  due  to  earth  currents  or  may  be  induced  cur- 
rents in  the  wire  due  to  atmospheric  conditions. 

Aurora  Australis. —  Auroral  displays  seen  in  the  southern  hemisphere 
about  the  south  pole  are  called  "aurora  australis." 

Cause  of  Aurora. — The  aurora  has  many  of  the  appearances  produced 
by  the  passage  of  electricity  through  rarefied  air  in  tubes. 

There  is  great  uncertainty  about  the  height  of  the  aurora  above  the 
earth. 

The  aurora  may  be  such  a  phenomenon  as  the  rainbow,  for  which 
every  vision  makes  its  own,  and  for  which  no  height  can  be  assigned. 
Various  estimates  and  alleged  measurements  of  the  height  vary  from 
half  a  mile  in  the  arctic  regions  to  forty  miles  above  the  earth,  and  in 
the  temperate  zone  for  the  lower  edge  of  the  arch  as  high  as  100  or  150 
miles. 

According  to  De  la  Rive  and  Marsh  and  Edlund's  theory,  the  aurora 
is  due  to  the  ascent  of  positive  electricity  to  the  upper  layers  of  the  air 
in  the  equatorial  regions  and  its  descent  in  a  zone  around  the  geograph- 
ical and  magnetic  poles  of  the  earth. 

The  magnetic  pole,  or  place  where  the  dipping-needle  would  be 
exactly  vertical,  is  not  in  the  same  position  as  the  geographical  pole, 
but  about  latitude  70°  05'  and  longitude  96°  46',  somewhere  in  the 
region  west  of  Hudson's  Bay. 

Magnetic  Elements.  —  The  difference  in  direction  of  a  magnetic 
needle  from  a  true  north  and  south  direction  is  the  magnetic  declina- 
tion. The  angle  a  dipping-needle  makes  with  a  horizontal  plane  is  the 
dip.  The  magnetic  force  usually  observed  is  the  horizontal  component  of 
the  force  and  is  called  the  "horizontal  intensity."  .  The  maximum  inten- 


OPTICAL  APPEARANCES.  139 

sity  is  in  the  direction  of  the  dip ;  it  is  equal  to  the  horizontal  intensity 
multiplied  by  the  secant  of  the  dip  :  the  intensity  is  expressed  in  dynes. 
The  dyne,  the  unit  of  force  on  the  centimetre-gramme-second  system 
of  units  (C.  G.  S.),  is  the  force  which,  acting  on  a  mass  of  one  gramme 
for  one  second  at  a  distance  of  one  centimetre,  will  impress  on  it  a 
velocity  of  one  centimetre  per  second.  The  declination  dip  and  inten- 
sity are  called  the  "  magnetic  elements."  They  have  daily,  monthly,  and 
annual  variations.  The  effect  of  the  moon  on  the  declination  is  about 
27"  in  middle  latitudes,  the  needle  making  two  oscillations  of  that 
amount  in  a  day.  The  daily  range,  apparently  due  to  temperature,  is 
about  15'.  It  is  less  on  a  cloudy  than  a  clear  day.  There  is  some 
indication  of  an  eleven-year  period  in  the  variations  of  the  elements 
corresponding  to  the  same  period  in  the  greatest  frequency  of  sun-spots. 
There  is  a  period  depending  on  the  time  of  rotation  of  the  sun. 

The  diurnal  range  of  magnetic  phenomena  is  not  the  same  in  the 
arctic  region  as  farther  south. 

There  is  no  known  relation  between  the  weather  and  variations  of 
the  earth's  magnetism. 

The  magnetic  elements  are  subject  to  slow  changes  extending  over 
great  lengths  of  time,  called  "secular  changes."  At  Washington  the 
magnetic  needle  pointed  51°  east  of  north  in  1792  and  4°  15'  to  the 
west  in  1889;  at  Paris,  in  1580,  it  pointed  11°  30'  east,  in  1666,  o°  08' 
west,  in  1814,  22°  34'  west,  since  which  time  the  declination  has  been 
diminishing,  and  in  1889  was  15°  32'  west. 

The  direction  a  freely  suspended  needle  would  assume  in  space  is 
derived  from  a  consideration  of  the  observed  values  of  declination  and 
dip  conjointly.  As  it  assumes  its  successive  directions,  it  describes  a 
conical  surface  with  the  pivot  of  the  needle  at  its  apex.  If  a  sphere  be 
described  with  its  centre  at  the  pivot  and  the  conical  surface  be 
extended  to  the  sphere,  the  line  of  intersection  of  the  two  will  graphi- 
cally represent  the  actual  secular  motion  of  the  needle.  The  values  of 
the  three  magnetic  elements  are  known  with  some  accuracy  for  the 
inhabited  portions  of  the  world  ;  for  a  few  places  the  rates  of  secular 
changes  in  the  elements  are  known  with  lesser  accuracy.  It  is  not 
known  whether  the  needle,  when  it  points  in  a  certain  direction  at  a 
given  place,  will  ever  return  to  the  same  position  again,  or  whether  it 


I4O  METEOROLOGY. 

will,  at  the  end  of  a  certain  period,  assume  the  same  direction  and  again 
sweep  over  the  same  path  in  the  same  period.  It  is  not  known  whether 
the  secular  variation  of  the  elements  has  a  period  or  not,  nor  whether 
if  there  shall  be  one  discovered  hereafter  it  will  be  the  same  for  all 
parts  of  the  world. 


CHAPTER  VII. 

WEATHER-MAPS. 

THE  condition  of  the  air  over  a  country  as  to  pressure,  temperature, 
etc.,  at  a  given  time  can  be  represented  graphically  on  a  map  of  the 
country  by  means  of  observations  of  these  various  conditions  made  at  a 
number  of  scattered  places.  Maps  showing  the  conditions  of  the  air 
and  the  state  of  weather  throughout  a  country,  by  conventional  signs 
by  lines  and  shading,  are  called  "  weather-maps." 

Isobars.  —  The  distribution  of  barometric  pressure  over  the  globe  is 
very  different  at  different  times.  On  weather-maps  pressures  reduced 
to  sea  level  are  generalized  and  graphically  represented  by  lines  through 
the  places  of  equal  pressure.  These  lines  are  called  "isobars."  Iso- 
bars are  usually  drawn  for  pressures  one-tenth  of  an  inch  apart,  for  the 
pressures  30.0  inches  30.1,  etc.,  29.9,  29.8,  etc.,  inches. 

Isotherms.  —  A  similar  graphical  representation  of  temperature  over 
a  country  without  reduction  to  sea  level,  by  lines  joining  places  of  equal 
temperature,  are  called  "  isotherms."  Isotherms  are  drawn  for  tempera- 
tures 10  degrees  apart,  30°,  40°,  50°,  etc.  On  the  United  States  weather- 
maps  the  isotherms  are  represented  on  the  same  map  with  the  isobars ; 
on  European  weather-maps  they  are  separate. 

Isohyetals.  —  A  graphic  representation  of  quantity  of  rainfall  by  lines 
through  places  having  equal  depths  of  rainfall  are  "isohyetals."  On 
weather-maps  rain  is  represented  by  shaded  areas. 

Cyclones,  Lows.  —  At  times  the  barometric  pressure  over  a  part  of  a 
country  is  much  below  the  average,  sometimes  as  low  as  29.0  inches  or 
even  less.  In  such  cases  the  pressure  increases  in  widening  circles  for 
a  distance  of  several  hundred  miles  from  the  place  of  lowest  pressure. 
A  system  of  isobars  of  this  kind  is  called  a  "  cyclone."  It  is  usually 
accompanied  by  rain  and  high  winds  in  the  country  over  which  it  lies. 

141 


142  METEOROLOGY. 

An  average  cyclone  ordinarily  covers  about  300,000  square  miles  of 
country,  frequently  much  less,  and  occasionally  very  much  more. 

The  lows  are  sometimes  called  storms.  The  centre  of  the  smallest 
isobar  is  called  the  storm  centre.  When  the  shape  of  the  isobars 
representing  an  area  of  low  pressure  are  not  rounding  nearly  circular,  the 
area  is  called  simply  a  "  low  "  or  a  "  depression." 

Anticyclones,  Highs.  —  At  times  the  barometric  pressure  over  an  area 
of  country  is  very  much  above  the  average,  sometimes  being  as  high  as 
31.0  inches,  or  even  more.  Areas  of  high  pressure  are  commonly  of 
much  greater  extent  than  lows.  When  the  map  is  sufficiently  large 
to  show  the  enclosed  isobars,  the  high  areas  are  shown  to  be  irregular, 
approximating  in  shape  irregular  triangles  with  rounded  corners.  These 
are  called  "  anticyclones,"  but  more  commonly  high  areas  or  simply 
"highs." 

The  area  of  the  earth's  surface  covered  in  the  United  States  by  a  high 
pressure  —  that  is,  a  pressure  greater  than  30.0  inches  and  increasing  to 
31.0  inches  or  so  —  is  sometimes  as  great  as  2,000,000  square  miles.  In 
Asia  they  occur  in  winter  4000  miles  from  west  to  east  and  3000  from 
north  to  south  within  the  3O.o-inch  isobar. 

Weather-Map.  — The  weather-map  issued  twice  a  day  by  the  Weather 
Bureau  shows  the  distribution  of  barometric  pressure  and  temperature 
over  the  whole  country  by  isobars  and  isotherms.  The  wind  direction 
is  represented  by  arrows  flying  with  the  wind. 

Rain  at  a  place  at  the  time  of  an  observation  is  represented  by  a 

black  circle  £  ,  clear  sky  by  a  light  circle  ?)  ,  snow  by  a  cross-barred 
circle  ^  ,  clouded  sky  by  a  heavy  ring  with  white  centre  &  ,  half- 
clouded  sky  by  ring  with  black  bar  (fc  .  The  observations  on  which 

these  maps  are  based  are  made  at  eight  o'clock  in  the  morning  and 
evening,  75th  meridian  time,  at  160  places  throughout  the  United  States 
and  the  Dominion  of  Canada.  The  observations  are  sent  to  the  central 
office  in  Washington  City  by  telegraph.  A  cipher  code  is  used  to  save 
time  and  expense  in  transmitting  messages.  All  the  weather  informa- 
tion from  a  station  is  comprised  in  five  or  ten  words.  Weather  mes- 
sages have  precedence  on  the  wires  over  all  other  telegraphic  business. 
When  cyclones  prevail  in  the  West  Indies,  reports  of  observations  are 


WE  A  THER-MAPS.  1 43 

received  from  Cuba,  Hayti,   San  Domingo,  and  Jamaica.      At  times, 
reports  are  received  from  the  Bermuda  Islands. 

The  printed  weather-map  based  on  observations  made  over  an  area 
of  more  than  three  and  a  half  millions  of  square  miles  of  the  earth's 
surface  is  issued  to  the  public  at  half-past  ten  o'clock  in  all  the  large 
cities  of  the  country,  with  forecasts  of  the  weather  expected  in  various 
parts  of  the  country  for  the  succeeding  twenty-four  hours. 

WINDS  IN  LOW  PRESSURE  AREA 


* 


>  GROUND  WINDS 

>  LOWER  CLOUDS 

>  UPPER   CLOUDS 

O  CALM 
FIG.  22. 

On  the  morning  weather-map  there  is  also  printed  the  stages  of  water 
at  a  number  of  places  on  the  principal  rivers  in  the  Mississippi  valley. 
The  stages  are  observed  on  the  respective  river  gauges  at  eight  o'clock 
each  morning.  Estimates  of  the  high  stages  of  water  expected  for 
several  days  ahead  are  given  when  the  stages  of  the  rivers  are  near 
the  flood-line. 

Warnings  are  sent  by  telegraph  from  the  central  office  in  Washington 


144  METEOROLOGY. 

to  various  places  on  the  lake  and  sea-coast  when  the  presence  of  a 
cyclone  indicates  the  possibility  of  high  winds  dangerous  to  shipping, 
also  warnings  of  cold  waves  or  tornadoes  to  places  where  they  are 
expected  to  occur. 

Baric  Law  of  Wind.  —  On  every  map  showing  a  cyclone,  the  winds 
over  the  region  covered  have  a  definite  direction  with  respect  to  the 
centre  of  low  pressure.  They  blow  spirally  inward  contrary  to  the 
direction  of  motion  of  the  hands  of  a  watch  lying  with  its  face  up. 
This  relation  of  wind  direction  to  isobar  is  called  the  "  baric  law  of  the 
wind."  From  a  theoretical  consideration  of  the  dynamics  of  the  air, 
and  the  deflecting  influence  of  the  earth's  rotation  on  currents  of  air 
over  the  surface  of  the  earth,  the  calculated  wind  directions  around  a 
low-pressure  area  are  found  to  agree  very  well  with  those  observed. 
Some  slight  local  deviations  are  produced  in  places  by  the  relief  of  the 
land,  hills,  etc. 

The  typical  observed  wind  directions  around  a  cyclone  centre  in 
middle  latitudes  are  shown  in  Fig.  22  for  the  surface  of  the  earth, 
at  the  height  of  the  lower  clouds  and  at  the  height  of  the  upper  clouds. 


GENERAL   VIEW    OF    CYCLONES    AND    HIGH-PRESSURE    AREAS. 

Direction  of  Winds  around  a  Low-Pr assure  Area.  —  In  a  cyclone  the 
wind  in  middle  latitudes  blows,  on  the  average,  approximately  in  a 
direction  half-way  between  the  tangent  to  the  isobar  and  the  radius 
drawn  towards  the  centre  of  the  cyclone.  East  of  the  centre,  the 
wind  is  inclined  more  toward  the  centre,  and  west  of  it  more  towards 
the  tangent.  The  average  angle  on  all  sides  to  the  tangent  is  in  the 
United  States  45°. 

The  direction  of  the  lower  clouds  is  nearly  tangent  to  the  isobars. 
The  motion  of  lower  cumulus  scud  is  inclined  outward  to  the  wind  at 
surface  of  the  earth,  14°. 5,  the  cirro-stratus  22°.8,  and  the  highest  true 
cirrus  2g°.6. 

The  upper  clouds  show  a  motion  of  the  air  out  from  the  centre  on 
all  sides,  but  very  much  more  so  on  the  east  side  than  the  west.  In 


WEATHER-MAPS.  145 

the  centre  of  a  cyclone  there  is  a  region  where  the  air  is  nearly  calm. 
Away  from  the  central  region  there  must  be  an  upward  component  of 
motion  to  the  air.  The  fact  that  the  wind  is  inclined  in  toward  the 
centre  of  the  cyclone  makes  it  certain  there  is  an  ascending  current. 

For  the  regions  near  the  equator  the  winds  at  the  edge  of  a  cyclone 
incline  in  towards  the  centre ;  near  the  centre,  they  are  more  in  the 
direction  of  the  tangent  to  the  isobars.  The  higher  the  latitude  the 
more  the  wind  inclines  outwardly  from  the  centre  of  the  cyclone,  owing 
to  the  deflection  produced  by  the  earth's  rotation  being  greater  in  high 
than  low  latitudes. 

On  the  west  side  of  cyclone  centres  in  the  United  States,  the  wind 
direction  is  more  inclined  towards  the  centre  than  in  the  case  of 
cyclones  in  Europe,  where  the  direction  is  neary  parallel  to  the  isobar  ; 
but  on  the  east  side  in  Europe  the  wind  is  much  more  inclined  to  the 
centre  than  it  is  in  cyclones  in  the  United  States. 

The  wind  direction  in  cyclones  is  more  toward  the  centre  in  the 
case  of  light  than  strong  winds. 

Height  of  South-east  Winds.  —  The  average  height  to  which  the 
south-east  wind  east  of  the  low  centre  extends  up  in  the  atmosphere  is 
only  half  that  of  the  north-west  wind.  The  wind  directions  do  not 
depend  on  the  direction  of  motion  of  the  low  centre. 

In  the  United  States  the  winds  to  the  north-west  and  south-east  of 
a  cyclone  centre  are  stronger  than  those  to  the  south-west  and  north- 
east. The  greatest  velocity  is  at  a  height  of  about  5000  feet.  At  the 
surface  of  the  earth,  a  few  feet  above  the  ground,  the  wind  velocity  is 
only  about  one-third  what  it  is  at  a  height  of  50  feet.  On  Mount 
Washington,  the  average  wind  velocity  in  a  cyclone  is  five  times  what 
it  is  at  50  feet  above  sea  level.  A  low  usually  extends  to  a  height 
of  10,000  feet ;  above  that  winds  from  the  west  prevail. 

The  lowest  pressure  on  Mount  Washington  follows  200  miles  or  5 
to  10  hours  behind  the  lowest  pressure  at  sea  level. 

An  approaching  cyclone  affects  the  currents  at  the  surface  of  the 
earth  earlier  than  at  a  height,  or  rather  the  winds  themselves  produce 
the  low  pressure  in  the  lower  layers  of  the  air. 

Pressure  Gradient.  —  The  more  numerous  the  isobars  in  a  given  dis- 
tance, the  greater  the  velocity  of  wind  in  the  region.  The  change  of 


146  METEOROLOGY. 

pressure  in  hundredths  of  an  inch  in  a  distance  of  500  miles  is  a  useful 
measure  of  pressure  gradient.  The  measure  is  often  taken  as  the 
number  of  millimetres  of  pressure  change  in  a  degree  of  latitude  equal 
to  69^  statute  miles. 

The  steepest  pressure  gradients  occur  to  the  north-west  and  south- 
east of  a  cyclone  centre  in  Europe,  and  mainly  to  the  north-west  of  the 
centre  in  the  United  States. 

Winter  and  Summer  Gradient. — The  same  pressure  gradient  in  winter 
or  in  the  night  corresponds  to  a  less  wind  velocity  than  in  summer  or 
in  the  daytime.  Gradients  and  surface-wind  velocities  in  the  United 
States  are  as  follows  at  8  A.M.  :  — 

Gradient  and  Wind  Velocity. 


PRESSURE  GRADIENT 
INCHES  IN  500  MILES. 

WIND  VELOCITY 
m  MILES  PER  HOUR. 

0-43 
0.48 

10 
IS 

0.52 

0.62 
0.76 

22 
30 

35 

The  wind  is  strongest  in  the  vicinity  of  the  centre  of  a  cyclone,  and 
diminishes  in  intensity  towards  the  edge.  From  3  P.M.  to  n  P.M.  the 
average  velocity  is  one-fifth  greater  than  the  average  for  the  rest  of 
the  day. 

In  a  cyclone,  the  forces  concerned  in  the  motion  of  the  air  are  :  the 
pressure  gradient  from  the  edge  of  the  cyclone  to  its  centre ;  the  cen- 
trifugal force  developed  by  the  rotation  of  the  air  around  the  centre, 
which  tends  to  drive  it  from  the  centre ;  the  deviating  force  due  to  the 
rotation  of  the  earth ;  the  resistance  due  to  inequalities  of  the  earth's 
surface  ;  the  friction  of  the  air  on  the  earth  ;  and  the  friction  of  the  air 
on  itself,  called  "viscosity."  Leaving  out  of  account  friction  and  viscos- 
ity, the  velocity  of  a  particle  of  air  due  to  pressure  gradient,  which  is  the 
main  element,  would  be  the  same  as  the  velocity  acquired  by  a  body  in 
rolling  down  an  inclined  plane  at  an  angle  corresponding  to  the  pressure 
gradient.  If,  for  instance,  the  centre  of  a  cyclone  is  at  Washington, 


WE  A  THER-MAPS.  1 47 

and  the  pressure  reduced  to  sea  level  is  29. 5  inches,  while  at  Chicago  it 
is  30.5,  also  reduced  to  sea  level,  then  the  velocity  due  to  this  gradient 
would  be  the  same  as  that  of  a  particle  moving  on  a  plane,  without  fric- 
tion, inclined  at  an  angle  equal  to  that  of  a  line  from  a  point  in  the  air 
above  Chicago  where  the  pressure  is  29.5  inches,  to  Washington  where 
the  pressure  is  the  same.  This,  with  the  air  at  a  temperature  of  30°, 
would  be  at  a  height  of  885  feet  above  Chicago.  The  distance  is  580 
miles. 

As  the  effects  of  friction  and  viscosity  are  not  known,  this  cannot  be 
used  in  computing  wind  velocity.  But  from  observed  velocities  and 
gradients,  the  effect  of  the  inequalities  of  the  earth  can  be  derived. 

Extent  of  Cyclones.  —  Cyclones  vary  greatly  in  the  extent  of  country 
over  which  they  occur.  The  distance  across  the  outside  isobar  is  some- 
times not  more  than  200  miles,  and  in  some  rare  cases  as  great  as 
1600. 

Clouds  in  Cyclones.  —  Over  a  region  covered  by  a  cyclone  the  sky  is 
cloudy.  Cirrus  cloud  precedes  a  coming  cyclone,  and  halos  are  visible. 
There  is  usually  also  rainfall  in  a  cyclone  ;  but  often  the  gyratory 
ascending  motion  of  the  air  produces  only  cloud,  which  drifts  away  or 
is  redissolved  in  the  air.  Back  of  a  cyclone  rounded,  sharp-edged, 
cumulus  clouds  follow.  The  average  cloudiness  diminishes  from  wholly 
clouded  sky  at  centre,  to  half  covered  sky  at  500  miles  from  centre. 

Cyclone  and  Rain.  —  Cyclones  usually  come  into  an  area  of  country 
under  observation  full-formed.  Sometimes  they  originate  over  an  area 
under  observation,  being  preceded  by  a  broad,  indefinite,  irregular  area 
of  pressure  slightly  below  the  average.  In  about  half  the  cases  of  for- 
mation, the  rain  in  the  area  of  a  cyclone  is  only  o.  I  of  an  inch  in  the 
first  twelve  hours  after  the  formation.  Sometimes  rain  does  not  occur 
for  twenty-four  hours  after  the  formation.  Usually,  however,  it  rains 
preceding  and  during  formation. 

Shape  of  Rain  Area. — The  shape  of  the  rain  area  accompanying 
cyclones  is  highly  irregular.  The  greatest  extent  of  rainfall  occurs 
usually  to  the  north-east  of  the  centre  of  a  cyclone  in  the  United  States. 
It  extends  around  the  low  on  all  sides,  but  farther  on  the  east  side 
than  the  west.  The  rain  area  sometimes  extends  700  miles  to  the  east 


148  METEOROLOGY. 

of  the  centre.     To  the  west  of  a  centre  of  a  low,  except  in  its  imme- 
diate vicinity,  the  rain  is  usually  lighter  than  to  the  east. 

The  diminution  of  temperature  upward  in  the  air  in  the  inner  circle 
of  a  cyclone  is  0.42  of  a  degree  per  hundred  feet,  as  shown  by  observa- 
tions at  Clermont-Ferrand,  and  at  the  top  of  the  Puy-de-Dome,  a 
mountain  in  France  where  the  difference  of  elevation  is  3543 
feet. 

Between  Denver,  5281  feet  above  sea  level,  and  the  summit  of  Pike's 
Peak,  14,134  feet,  a  difference  of  8853  feet,  in  the  case  of  areas  of 
pressure  as  low  as  29.6  inches,  the  difference  of  temperature  at  5  hours 
and  7  minutes  A.M.  is  30.4  degrees ;  at  i  hour  and  7  minutes  P.M.,  36.6 
degrees  ;  at  9  hours  7  minutes  P.M.,  35.4  degrees.  In  case  of  pressures 
as  high  as  30.6  inches,  the  difference  in  the  temperatures  at  the  same 
times  as  given  above  are  15.6  degrees,  36.8  degrees,  and  25.6  degrees. 
The  average  rate  of  diminution  for  100  feet,  at  1.07  P.M.,  is  0.415  of  a 
degree  for  both  lows  and  highs. 

Between  Portland,  Me.,  and  Mount  Washington,  6279  feet,  the  dif- 
ference of  temperature  in  lows,  at  7  A.M.,  3  P.M.,  and  10  P.M.,  are 
at  the  rates  0.281,  0.486,  and  0.443  °f  a  degree  in  100  feet.  In  highs, 
the  rates  at  the  same  hours  are  0.146,  0.205,  and  0.143  °f  a  degree. 
The  proximity  of  sea  in  this  case  may  exercise  a  modifying  influence. 

Movement  of  Cyclone.  —  A  system  of  cyclonic  isobars  has  a  proper 
motion  over  the  surface  of  the  earth.  In  the  United  States,  the  centres 
move  generally  in  an  easterly  or  north-easterly  direction  with  a  velocity 
of  25  miles  an  hour  on  the  average.  The  velocity  at  different  times 
in  different  cyclones  varies  from  15  to  60  miles  an  hour.  While  the 
most  usual  direction  of  motion  is  north-east  or  east,  centres  do  move 
occasionally  in  other  directions,  south,  south-east,  and  sometimes  directly 
north.  Very  rarely  they  move  north-west.  Only  in  the  tropics  is  the 
direction  of  motion  of  a  cyclone  towards  the  west  or  slightly  south  of 
west.  Those  moving  from  the  south-west  are  only  one-sixth  of  the  num- 
ber moving  from  the  north-west.  Sometimes  lows  originate  in  the 
north-west,  and  after  moving  south-east  turn  and  move  to  the  north-east. 
At  the  turning-point  the  motion  is  slow.  The  direction  of  motion  of 
lows  is  that  of  the  general  circulation  of  the  air.  The  motion  north 


WE  A  THER-MAPS.  1 49 

is  more  rapid  than  south.  Those  moving  north  extend  to  a  great  height 
in  the  air,  and  are  controlled  by  the  upper  air  currents. 

In  moving  from  the  Gulf  of  Mexico  to  New  England,  the  pressure 
at  the  centre  of  a  cyclone  diminishes,  on  the  average,  about  0.3  of  an 
inch.  A  low  moving  directly  east  from  Lake  Superior  to  the  Gulf 
of  St.  Lawrence  often  diminishes  much  more.  It  is  a  characteristic  of 
the  lows  moving  from  the  Gulf  of  Mexico  north,  that  they  are  accom- 
panied by  heavy  rains. 

The  low  areas  can  sometimes  be  traced  a  long  distance  over  the 
earth's  surface  before  breaking  up,  sometimes  as  much  as  half-way 
around  the  globe. 

There  is  a  diurnal  inequality  perceptible  in  their  motion.  The 
average  velocity  from  7  A.M.  to  4  P.M.  is  25.9  miles  per  hour;  from  4  P.M. 
to  ii  P.M.,  31.9  miles. 

The  average  rate  of  motion  of  those  crossing  the  Atlantic  Ocean  is 
19.6  miles  per  hour,  somewhat  less  than  on  land. 

The  tendency  is  for  lows  to  move  in  the  direction  of  the  heaviest 
rain  to  the  east  of  them.  With  very  heavy  rains  accompanying  them, 
the  motion  is  slower  than  with  light  rains. 

When  a  centre  of  low  pressure,  29.5  inches,  leaves  the  United  States 
the  chances  are  only  one  out  of  nine  that  it  will  pass  over  any  part  of 
Great  Britain.  The  probability  that  it  will  give  rise  to  a  gale  anywhere 
on  the  English  coast  is  only  one  in  six ;  the  probability  of  a  very  fresh 
breeze  is  one  in  two. 

The  isobars  of  a  cyclone  are  usually  elliptical  in  shape.  In  55  per 
cent  of  the  cases  the  major  axis  exceeds  the  minor  by  \  its  length; 
in  30  per  cent  it  is  more  than  double  the  minor ;  in  9  per  cent,  3  times 
the  minor;  and  in  4  per  cent,  4  times  the  minor. 

The  prevailing  direction  of  the  long  axis  is  N.  40°  E. 

Changing  Shape  of  Cyclone.  —  Asa  cyclone  moves,  it  is  attended  in 
the  regions  over  which  it  passes  by  its  characteristic  winds.  Cyclones 
are  subject  to  great  changes  in  the  shape  of  the  isobars  as  they  progress. 
The  isobars  may  change  from  nearly  circular  to  decidedly  oval  in  shape ; 
the  area  of  country  covered  may  increase  as  they  progress ;  the  distance 
between  the  isobars  may  diminish ;  sometimes  the  isobars  become 
irregular  and  all  resemblance  to  a  cyclone  is  lost.  There  is  a  charac- 


150  METEOROLOGY. 

teristic  increase  in  the  distances  between  the  successive  isobars  from  the 
centre  of  a  cyclone  outward  in  cases  where  the  winds  are  strong,  corre- 
sponding to  the  deeper  depressions. 

Regular  Isobars.  —  Areas  of  low  pressure,  with  the  pressure  at  cen- 
tre 29.3  inches  or  less,  usually  have  regular  isobars.  In  middle 
latitudes  extensive  areas  of  low  pressure  of  various  shapes  often  occur 
without  the  isobars  being  regular  or  approximately  concentric  as  in  the 
case  of  cyclones.  In  low-pressure  areas  in  the  tropics,  only  regular- 
shaped  cyclonic  isobars  occur.  Regular  circular  isobars  indicate  that 
great  depth  of  air  is  concerned  in  the  motions  of  a  cyclone. 

Height  of  Wind.  —  In  cyclones  of  small  diameter  the  circulating  winds 
do  not  extend  to  a  greater  height  in  the  air  than  6000  feet.  When  the 
diameter  of  a  cyclone  is  great,  the  wind  system  may  reach  to  a  very 
great  height  in  the  air. 

Cyclones  with  very  low  central  pressure  occur  mostly  in  winter ; 
occasionally,  however,  such  a  one  occurs  in  summer. 

Shape  of  Low-Pressure  Area.  —  In  the  United  States  the  areas  of  low 
barometric  pressure  are  usually  of  irregular  shape.  The  cyclonic  form 
is  rather  the  exception  than  the  rule.  On  account  of  the  limited  area 
of  the  earth's  surface  taken  in  by  the  weather-maps,  the  isobars  of 
lows  are  in  many  cases  not  closed  or  apparently  not  continuous.  In 
four-fifths  of  all  the  lows  with  closed  isobars  the  shape  of  the  low- 
pressure  area  is  oval,  with  the  long  diameter  lying  from  south-west  to 
north-east,  and  about  twice  as  long  as  the  short  diameter  from  north- 
west to  south-east.  In  one-fifth  of  the  cases  the  long  diameter  is  3  or 
4  times  the  short  one.  Lows  with  open  isobars  may  be  open  in  any 
direction. 

At  times  lows  occur  with  highs  from  400  to  1000  miles  distant. 
A  high  area  to  the  east  or  south-east  of  a  low  has  no  significance 
as  regards  coming  weather  in  the  United  States.  Any  action  there 
may  be  between  a  low  and  a  high  only  takes  place  when  the  high 
is  to  the  north,  north-west,  west,  or  south-west  of  the  low. 

Double  Lows.  —  Sometimes  a  low  occurs  to  the  north  over  the  Lake 
region,  and  one  to  the  south  in  Louisiana,  presenting  a  double  V-shaped 
appearance,  with  a  high  to  the  north-west.  Double  lows  also  occur, 
one  in  Colorado  and  the  other  in  the  Lake  region ;  and  at  times  one 


WEATHER-MAPS.  151 

in  the  Lake  region  and  another  on  the  Atlantic  coast  about  New 
England. 

Cause  of  Oval  Shape.  —  In  long  oval  lows  with  a  high  to  the  north- 
west, the  winds  on  the  north-west  of  the  centre  are  stronger  than  the 
inflowing  winds  on  the  south-east,  possibly  due  in  part  to  the  greater 
effect  of  the  earth's  rotation  in  deviating  the  northern  winds,  as  com- 
pared-with  the  same  effect  on  the  winds  more  to  the  south  in  a  lower 
latitude. 

Depth  of  Depression.  —  The  amount  of  barometric  depression  in  the 
centre  of  a  low  varies  at  different  times.  In  the  north  it  is  greater  on 
the  average  than  in  the  south.  The  depth  of  depression  has  no  relation 
to  the  amount  of  rainfall.  Even  the  slightest  depressions  that  first 
appear  in  the  vicinity  of  the  Gulf  of  Mexico  are  apt  to  be  accompanied 
by  great  rainfall. 

All  the  lows  that  appear  in  the  United  States  may  be  divided  accord- 
ing to  the  paths  they  pursue  into  eleven  classes.  These  are  as  follows 
in  the  order  of  their  relative  frequency  :  — 

I.  Lows  that  first  appear  to  the  north  of  North  Dakota  and  Montana 
and  move  directly  east  to  the  Gulf  of  St.  Lawrence. 

II.  Lows  that  originate  north  of  North  Dakota  and  Montana  and 
move  slightly  south  of  east  to  the  region  of  the  Great  Lakes,  and  then 
turn  and  move  north-east  toward  the  Gulf  of  St.  Lawrence. 

III.  Lows  that  originate  north  of  North  Dakota  and  Montana  and 
move  south-east  to  Nebraska  and  Kansas,  and  then  turn  and  move 
north-east  to  the  region  of  the  Great  Lakes. 

IV.  Lows  that  originate  north  of  North  Dakota  and  Montana  and 
move  south-east  to  the  Gulf  of  Mexico,  or  vicinity,  and  then  turn  and 
move  north-east  across  the  country  to  the  Gulf  of  St.  Lawrence. 

V.  Lows  that  originate  in  Colorado  or  thereabout  and  move  north-east. 

VI.  Lows  that  appear  first  in  Colorado  and  move  directly  east. 

VII.  Lows  that  appear  first  in  Texas  and  move  north-east. 

VIII.  Lows  that  appear  first  in  Texas  and  move  east  to  the  Atlantic 
coast,  and  then  north-east. 

IX.  Lows  that  come  from  the  Gulf  of  Mexico  and  move  north. 

X.  Lows  that  come  from  the  Gulf  of  Mexico  and  move  north-east. 


152 


METEOROLOGY. 


XI.  Lows  that  appear  on  the  North  Carolina  coast  and  move  north 
inland  a  few  hundred  miles,  then  turn  and  move  north-east. 

On  very  rare  occasions  a  low  will  move  north-west.  A  case  of  this 
kind  occurred  March  3,  1881. 

The  paths  of  these  areas  are  shown  on  the  accompanying  chart, 
Fig.  23. 

CYCLONE  PATHS  IN  THE  UNITED   STATES. 


125       120 115       110      106      100     95      90       85      80      75       70   ^O^"  03 


FIG.  23. 


The  following  table  gives  the  numbers  of  these  various  classes  of 
lows  that  have  appeared  in  the  different  months  in  10  years,  and  the 
velocities  of  each  class  and  for  all  the  classes  for  each  month. 


WE  A  THER-MAPS. 


153 


NUMBER  OF  Lows  OF  DIFFERENT  CLASSES  IN  THE  UNITED  STATES,  1882  TO  1891,  AND 
AVERAGE  VELOCITY  IN  MILES  PER  HOUR. 


I. 

II. 

III. 

IV. 

V. 

VI. 

VII. 

VIII. 

IX. 

X. 

XI. 

Total 

Number 

Mean 
Velocity 

January 

6 

6 

23 

7 

3 

II 

6 

I 

63 

40.0 

February 

7 

8 

16 

3 

6 

3 

2 

13 

2 

2 

62 

38.3 

March  . 

7 

7 

16 

i 

ii 

i 

5 

8 

6 

5 

2 

69 

33-o 

April     . 

3 

7 

6 

16. 

5 

2 

7 

i 

3 

5° 

28.7 

May 

8 

5 

7 

12 

6 

2 

6 

2 

I 

49 

26.6 

June      . 

9 

2 

3 

9 

3 

3 

i 

2 

I 

33 

24.9 

July      . 

8 

II 

3 

ii 

6 

2 

i 

2 

I 

45 

24.9 

August  . 

ii 

9 

8 

8 

3 

I 

3 

i 

2 

I 

47 

28.1 

September 

ii 

7 

7 

i 

8 

2 

2 

3 

I 

42 

25-4 

October 

ii 

ii 

7 

2 

7 

3 

5 

2 

5 

I 

54 

29.2 

November 

10 

7 

7 

I 

ii 

i 

I 

9 

4 

I 

52 

32.3 

December 

25 

5 

ii 

10 

13 

5 

2 

7i 

36.6 

116 

85 

114 

8 

116 

36 

15 

81 

21 

36 

9 

637 

Total    . 

31-1 

33-2 

34-6 

35-6 

30-5 

30.8 

27.1 

32.5 

27.1 

29.0 

24.2 

31-7 

In  Russia  the  average  velocity  of  cyclone  centres  is  26  miles  an  hour 
in  winter,  25  in  spring,  21  in  summer,  and  24  in  autumn.  In  Russia 
the  motion,  the  second  day  after  the  appearance  of  a  cyclone,  is  eight- 
tenths  what  it  is  the  first,  the  third  day  six-tenths,  and  the  fourth  day 
four-tenths. 

For  low-pressure  areas,  with  the  centres  in  Illinois  and  the  isobars 
nearly  regular  and  circular  and  the  pressure  at  least  as  low  as  29.3 
inches  at  the  centre,  the  accompanying  chart  Fig.  24  shows  the  aver- 
age velocity  of  the  wind  in  different  parts  of  the  country,  at  8  A.M.,  as 
derived  from  ten  selected  cases. 


154 


METEOROLOGY. 


CENTRE  OF  LOW-PRESSURE  AREA  IN  ILLINOIS. 
AVERAGE  WIND  VELOCITY  IN  MILES  PER  HOUR. 


,  FIG.  24. 

The  greatest  wind  velocity  in  miles  per  hour  at  8  A.M.  in  different 
parts  of  the  country  are  shown  on  the  chart  Fig.  25  below,  for  a  regu- 
lar low-pressure  area  in  Illinois,  with  the  pressure  at  the  centre  29.3 
inches,  selected  from  ten  such  cases. 

In  the  following  tables  are  given  the  statistics  of  the  various  charac- 
teristics of  areas  of  low  air  pressure  that  have  appeared  in  the  United 
States  and  vicinity  in  the  twenty  years  from  1870  to  1890,  as  obtained 
from  the  daily  weather-maps  of  the  United  States. 

Table  I.  shows  the  number  of  low  areas  that  have  appeared  in  differ- 
ent parts  of  the  country,  the  lowest  pressure  at  the  centre,  and  whether 
the  pressure  was  rising,  falling,  or  stationary  at  the  centre,  as  compared 
with  the  pressure  at  centre  on  the  day  following. 

Table  II.  shows  the  number  of  low  areas  appearing  in  different 
months,  and  their  direction  of  motion  for  the  24  hours  following. 


WE  A  THER-MAPS. 

CENTRE  OF  LOW  PRESSURE  IN  ILLINOIS. 
GREATEST  WIND  VELOCITY  IN  MILES  PER  HOUR. 


155 


FIG.  25. 

Table  III.  shows  the  number  of  low  areas  occurring  in  different 
months,  and  the  lowest  pressure  at  the  centre  of  the  areas. 

Table  IV.  shows  the  number  of  low  areas  of  pressure,  and  the 
number  of  cases  in  which  the  rise  or  fall  of  pressure  at  the  centre  in 
24  hours  was  o.i  in.,  0.2  in.,  0.3  in.,  0.4  in.,  etc. 

Table  V.  shows  the  direction  of  motion  of  the  low  area,  with  the  high 
area  in  different  positions  with  respect  to  the  centre  of  the  low. 


156 


METEOROLOGY. 


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WEATHER-MAPS. 


157 


TABLE   II. —NUMBER  OF  CASES   OF   LOW-PRESSURE  AREAS  AND 
DIRECTION  OF  MOTION. 


N.  B. 

E. 

S,  B. 

S. 

s  .  w. 

w. 

N.  W. 

w. 

STAT. 

January      .     .     . 

216 

41 

26 

9 

3 

I 

4 

9 

8 

February    .     .     . 

213 

53 

42 

5 

6 

6 

4 

March  .... 

233 

46 

54 

ii 

i 

i 

23 

II 

April     .... 

180 

61 

49 

5 

4 

5 

22 

12 

May      .... 

130 

4i 

38 

6 

3 

I 

3 

14 

14 

June      .... 

76 

24 

23 

i 

2 

4 

3 

7 

10 

Tulv. 

•»•» 

JO 

IQ 

j 

2 

c 

I 

jj 

trf 

*y 

J 

August  .... 

45 

25 

15 

3 

5 

5 

8 

21 

2 

September      .     . 

74 

35 

30 

2 

2 

3 

ii 

27 

18 

October     .     .     . 

100 

74 

22 

6 

I 

2 

6 

42 

7 

November  .     .     . 

121 

101 

24 

4 

3 

36 

10 

December  .     .     . 

137 

100 

22 

2 

2 

2 

5 

39 

I 

Year  .     .     . 

I558 

641 

364 

57 

24 

18 

57 

251 

98 

3068 

Per  cent  .     . 

SI 

21 

12 

2 

I 

I 

2 

8 

2 

158 


METEOROLOGY. 


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WE  A  THER-MAPS. 


159 


TABLE   IV.  —  NUMBER  OF  CASES  OF  LOW-PRESSURE  AREAS    WITH 
CHANGE  OF  PRESSURE  AT  THE  CENTRE  IN  24  HOURS. 


PRESSURE   AT   CENTRE  OF 
LOW  AREA. 

CHANGE  OF  PRESSURE  IN   24   HOURS   AND  NUMBER  OF  CASES. 

f 

0 

+  .1  in. 

+  .2  in. 

+  .3  in. 

+  .4  in. 

+  .5  in. 

+  .6  in. 

-f  -7in- 

16 

10 

19 

13 

10 

10 

5 

15 

29.2  in.  and  less. 

—  .1 

-.2 

-•3 

-•4 

-•5 

-.6 

-•7 

I 

9 

3 

X 

i 

i 

z 

o 

+  .1 

+  .2 

+  •3 

+  4 

+  •5 

+  .6 

+  •7 

36 

50 

44 

46 

18 

8 

8 

5 

29.3  in.     29.4  in. 

—  .1 

—  .2 

-•3 

-•4 

-•5 

-.6 

-•7 

30 

21 

10 

zo 

5 

o 

+  .1 

+  .2 

+.3 

+  4 

+  •5 

+  .6 

+  •7 

126 

112 

103 

64 

17 

7 

2 

29.5  in.     29.6  in. 

—  .1 

2 

-•3 

-•4 

-•5 

-.6 

-•7 

82 

52 

45 

23 

9 

2 

5 

f 

o 

+  .1 

+  .2 

+  •3 

+  4 

+  •5 

+  .6 

+  •7 

231 

157 

55 

26 

7 

i 

29.7  in.     29.8  in. 

—  .1 

—  .2 

-•3 

-•4 

-•5 

-.6 

-•7 

161 

114 

68 

32 

14 

13 

iz 

TABLE   V.  —  NUMBER   OF   CASES   OF    LOW-PRESSURE    AREAS    AND 
DIRECTION  OF  MOTION  IN  RELATION  TO  HIGH  PRESSURE. 


N.  E. 

E. 

S.  E. 

S. 

S.  W. 

w. 

N.  W. 

N. 

High  to  s.  w.  of  low     .... 
Per  cent 

I67 
eg 

64 

22 

21 

2 
I 

2 

I 

2 

I 

2 

I 

27 

High  to  n.  w.  of  low    .... 

J47 
67 

31 
14 

19 

4 

2 

O 

I 

2 

I 

H 

High  to  n.  e.  of  low     .... 
Per  cent       .                ... 

179 

« 

41 

12 

33 
10 

5 
j 

5 

2 

5 

2 

15 

53 
16 

77 

re 

23 

•j 

c 

o 

6 

Per  cent 

*•* 

02 

Id. 

2 

Q 

160  METEOROLOGY. 

The  conclusions  that  may  be  drawn  from  the  weather-maps,  in  regard 
to  the  low-pressure  areas  in  the  United  States,  are  as  follows  :  — 

In  the  case  of  regular  lows,  with  nearly  circular  isobars,  which  form 
about  10  per  cent  of  all  the  lows  appearing,  the  motion  in  6  per  cent  of 
the  cases  is  north  ;  in  53  per  cent,  north-east ;  in  31  per  cent,  east ;  and 
in  10  per  cent,  south-east. 

In  the  case  of  regular  lows,  with  nearly  circular  isobars,  where  the 
pressure  at  centre  is  29.2  inches  or  less,  the  velocity  of  the  low  centre 
is,  on  the  average,  564  miles  per  day :  where  the  pressure  at  the  centre 
is  29.6  inches,  the  average  velocity  is  660  miles  per  day. 

The  velocity  of  motion  of  low-pressure  areas  in  different  directions  in 
the  interval  of  a  day  are,  for  the  north-east,  704  miles  ;  east,  593  ;  south- 
east, 572  ;  south,  385  ;  south-west,  345  ;  west,  334 ;  north-west,  389 ;  and 
west,  456. 

Of  all  the  low  centres  visible  on  two  successive  days,  and  moving  east- 
ward, 48  per  cent  preserve  the  same  direction  of  motion  on  the  two 
days ;  30  per  cent  have  the  direction  of  motion  on  the  second  day  forty- 
five  degrees  different  from  that  of  the  first ;  22  per  cent  have  the  direc- 
tion of  motion  on  the  second  day  ninety  degrees  or  more  different  from 
the  first. 

Of  all  the  low  centres  of  motion  visible  on  two  successive  days,  the 
direction  of  motion  on  the  two  days  is  as  follows  :  — 


2  per  cent  north  and  north. 

3  per  cent  north  and  north-east. 

3  per  cent  north-east  and  north. 

32  per  cent  north-east  and  north-east. 
6  per  cent  north-east  and  east. 

4  per  cent  north-east  and  south-east. 


9  per  cent  east  and  east. 

3  per  cent  east  and  south-east. 

1 6  per  cent  south-east  and  north-east. 
5  per  cent  south-east  and  east. 

4  per  cent  south-east  and  south-east, 
i  per  cent  south-east  and  south. 


12  per  cent  east  and  north-east. 

In  all  the  cases  of  low  pressure,  the  pressure  diminishes  on  two  suc- 
cessive days  in  40  per  cent  of  the  cases  ;  in  34  per  cent  of  all  cases,  the 
pressure  diminishes  for  one  day  and  increases  on  the  next ;  in  10 
per  cent  of  the  cases,  the  pressure  increases  on  two  successive  days  ;  in 
1 1  per  cent  of  the  cases,  the  pressure,  increasing  one  day,  diminishes 
the  next ;  in  5  per  cent  of  the  cases,  the  pressure  remains  the  same, 
within  less  than  than  o.  i  of  an  inch,  on  two  successive  days. 


WE  A  THER-MAPS. 


161 


Of  all  cases  where  the  pressure  is  diminishing,  in  54  per  cent  of  cases 
it  diminishes  also  the  next  day,  and  in  46  per  cent  increases. 

Of  all  cases  where  the  pressure  is  increasing,  in  47  per  cent  of  the 
cases  it  increases  the  next  day,  and  in  53  per  cent  diminishes. 

On  the  first  appearance  of  a  low  pressure,  in  the  24  hours  succeeding 
there  is  no  change  in  the  pressure  at  centre  in  21  per  cent  of  the 
cases  ;  in  23  per  cent  of  the  cases  there  is  an  increase  in  the  pressure 
at  centre,  and  in  56  per  cent  a  diminution. 

On  the  second  day  of  the  appearance  of  a  low  pressure,  in  the  24 
hours  succeeding  there  is  no  change  in  the  pressure  at  centre  in  21 
per  cent  of  the  cases  ;  in  42  per  cent  of  the  cases  there  is  an  increase 
of  the  pressure  at  centre,  and  in  37  per  cent  a  diminution. 

In  the  case  of  an  appearance  of  an  area  of  pressure  with  the  central 
pressure  29.6  inches,  the  chances  are  about  equal  that  there  will  be  an 
increase  or  decrease  of  the  pressure  at  the  centre  the  next  day  ;  for 
greater  pressure  it  is  more  likely  the  pressure  will  decrease,  and  for  less 
pressure  increase 

AREA    OF    HIGH    PRESSURE. 

In  an  area  of  high  pressure,  the  winds  blow  from  the  centre  outward, 
and  the  paths  are  slightly  curved,  as  shown  in  Fig.  26.  They  are 
opposite  in  direction  to  the  winds  of  a  low-pressure  area. 

The  winds  in  a  high-pressure  area 
are  usually  light.  At  times,  however, 
the  winds  may  be  very  strong  when 
blowing  from  the  north-west  in  a  case 
where  there  is  a  low  area  to  the  south- 
east of  a  high-pressure  area,  closely 
adjoining,  or  there  may  be  very  strong 
north-east  winds  from  it,  in  the  case 
of  a  low-pressure  area  to  the  south- 
west of  it. 

In  the  centre  of  a  high  pressure  it 
is  calm.  The  winds  increase  in  veloc- 
ity from  the  centre  outward. 

The  intense  high-pressure  area  is  essentially  an  occurrence  of  winter 


WINDS  TN   HIGH   PRESSURE  AREA. 


>  LOWER  WINDS. 

>  UPPER  WINDS, 

FIG.  26. 


162  METEOROLOGY. 

time  and  of  high  latitudes.  More  than  nine-tenths  of  all  the  highs, 
with  pressures  at  centre  30.7  inches  or  more,  in  the  United  States, 
occur  from  October  to  April.  The  high  areas  of  summer  rarely  have 
the  greatest  pressure  much  above  30.3  inches. 

Over  a  country  covered  by  a  high  pressure  the  sky  is  usually  clear. 
Very  little  is  known  with  certainty  about  the  wind  direction  in  the 
upper  air  in  high  areas,  there  being  commonly  no  clouds  visible  by 
which  the  direction  can  be  observed. 

Motion  of  Highs. — Areas  of  high  pressure  move  generally  south  or 
south-east,  sometimes  east.  The  motion  is  often  of  a  desultory,  uncer- 
tain character.  The  shape  of  the  isobars  changes  a  good  deal  as  the 
high  moves,  and  the  centres  often  cannot  be  located  closely. 

The  high  pressures  of  the  United  States  originate  for  the  most  part 
to  the  north  of  latitude  49°  in  the  far  north-west,  and  move  down  over 
the  United  States  in  a  south-easterly  direction.  A  few  originate  north 
of  the  Great  Lakes,  and  move  south-east  or  east ;  a  few  form  over  the 
Rocky  Mountain  region,  and  move  generally  east.  In  many  cases 
highs  originate  in  the  north-west,  after  the  wind  has  blown  40  miles  an 
hour  from  the  north. 

Velocity  of  High  Pressures.  —  The  average  velocity  of  a  high-pres- 
sure centre  is  about  25  miles  an  hour ;  a  little  greater  in  winter,  27 
miles;  a  little  less  in  summer,  21  miles.  Usually  there  is  in  winter  a 
light  fall  of  snow  on  the  south-east  border  of  a  high  area,  equivalent  to 
a  depth  of  rainfall  of  about  o.i  of  an  inch.  Over  the  central  part  of 
the  United  States,  highs  usually  do  not  continue  more  than  three  days. 
Over  central  Europe,  a  high  will  sometimes  last  for  two  weeks  or 
more. 

Temperature  with  Ascent.  —  At  the  centre  of  a  high  pressure  there 
is  at  times  an  increase  of  temperature  with  ascent  in  the  air. 

In  the  high  of  November  11-25,  1889,  over  Europe,  there  was  an 
increase  of  temperature  upward  in  the  air  at  the  rate  of  0.07  of  a 
degree  per  hundred  feet.  This  is  probably  not  generally  the  case  in 
high  areas  ;  when  the  temperature  in  a  high-pressure  area  is  low  at  the 
ground,  it  is  usually  correspondingly  low  at  a  height,  as,  for  instance, 
in  the  case  of  the  high  area  of  30.5  in  Colorado,  on  December  21,  1886; 
when  it  was  —  18°  at  Denver  it  was  —32°  on  Pike's  Peak.  The  inver- 


WE  A  THER-MAPS,  1 63 

sion  of  temperature,  or  increase  with  ascent,  in  a  high  has  been  ascer- 
tained in  Italy  to  extend  only  to  a  height  of  2300  feet. 

There  is  but  little  vapour  in  the  air  in  a  high-pressure  area. 

Temperature  in  High  and  Low  Pressure  Areas.  — In  the  region 
covered  by  a  cyclone,  the  isotherms  passing  through  it  are  crowded 
together  towards  the  centre  ;  in  an  area  of  high  pressure,  they  are 
spread  out.  This  is  shown  on  the  map,  page  190. 

At  the  centre  of  a  cyclone,  and  to  the  east  of  the  centre,  the  tem- 
perature is  relatively  high  ;  in  a  high-pressure  area,  the  temperature  is 
low.  The  general  air  current  in  a  cyclone  in  the  United  States  is  from 
the  south,  and  in  an  area  of  high  pressure  from  the  north.  These 
winds  carry  with  them,  in  a  measure,  the  temperatures  peculiar  to  the 
regions  from  which  they  blow. 

Pressure  Gradient  Above. — The  outward  pressure  gradient  dimin- 
ishes with  ascent  in  the  air  over  a  region  covered  by  a  cyclone.  At 
a  height  which  is  not  known,  but  is  considerably  over  10,000  feet,  the 
gradient  is  even  supposed  to  be  reversed,  and  the  pressure  increases 
from  the  edge  toward  the  place  vertically  above  the  lowest  pressure  at 
the  ground. 

There  may,  however,  be  outward  motion  of  air  at  a  height  from  the 
centre  towards  the  edge  even  with  the  pressure  diminishing  towards 
the  centre  the  same  as  at  the  ground.  In  a  high  pressure  the  inward 
pressure  gradients  also  diminish  with  ascent,  and,  it  is  supposed,  be- 
come reversed  at  a  great  height.  There  are  no  actual  observations, 
however,  to  indicate  this,  but  it  is  inferred  from  the  observed  direction 
of  the  winds  as  shown  by  clouds,  the  gradients  being  assumed  according 
to  the  wind  direction.  The  excess  of  pressure  at  centre  above  the  sur- 
rounding pressures  is  still  perceptible  at  a  height  of  10,000  feet. 

Circulation  in  High  and  Low.  —  There  may  be  a  partial  circulation 
of  the  air  from  the  cyclone  to  a  neighbouring  high  or  the  region  of 
relatively  high  pressure  around  a  cyclone  in  the  upper  air,  and  from  the 
high  to  the  cyclone  in  the  lower  air.  This,  however,  is  not  necessarily 
always  the  case,  but  is  often  so. 

Axis  of  Cyclone.  —  A  line  joining  the  centre  of  a  cyclone  at  the 
earth's  surface  with  the  centres  in  the  air  above  at  greater  and  greater 
heights  is  called  the  "axis  of  a  cyclone"  ;  this  line  is  inclined  backward 


1 64  METEOROLOGY. 

toward  that  high-pressure  area  which  is  usually  back  of  it.  On  the 
west  side,  the  air  being  so  much  colder  than  on  the  east  side,  the 
pressure  diminishes  more  rapidly  with  ascent,  and  the  centres  of  low 
pressure  in  successive  layers  must  be  farther  and  farther  towards 
the  region  of  cold.  There  is  a  vertical  component  of  motion  of  the  air  in 
a  cyclone  upward,  and  in  a  high  pressure  motion  of  the  air  downward. 

Cyclone  Motion  and  Rainfall.  —  The  centre  of  a  cyclone  moves 
approximately  towards  the  place  of  greatest  rainfall  in  the  preceding 
24  hours.  The  influence  of  rain,  however,  in  directing  the  motion  is 
obscure  and  irregular. 

Cyclones  and  Mountains.  —  Mountain  regions,  in  spite  of  the  pre- 
dominating rain,  have  a  less  number  of  cyclone  centres  passing  over 
them  than  equal  areas  of  sea  or  of  land  where  there  are  no  mountains. 
This  shows  that  mountain  rainfall  is  largely  due  to  the  relief  features  of 
the  country,  and  not  to  cyclonic  ascension  of  air. 

Velocity  and  Depth.  —  The  velocity  of  progress  of  the  centre  of  a 
cyclone  increases  with  the  depth  of  pressure  at  the  centre. 

Motion  of  Cyclone  and  Circulation  of  Air.  —  The  progressive  motion 
of  the  centre  of  a  cyclone  is  in  the  direction  of  the  air  current  of 
greatest  energy,  or  in  the  direction  of  the  general  circulation  of  the 
air  at  a  place.  As  the  condition  of  motion  at  different  altitudes  in 
the  whirl  is  different,  it  is  not  the  condition  of  motion  in  the  lower 
air,  but  that  of  the  average  of  all  the  layers  of  air,  that  determines 
the  motion  of  the  centre. 

In  the  tropics  the  westerly  progression  of  cyclones  occurs  almost 
wholly  at  a  season  when  the  westerly  velocity  of  the  air  extends  up 
to  the  greatest  height. 

There  is  a  diurnal  period  in  the  development  and  progress  of  the 
low-pressure  areas :  the  motion  is  more  rapid  toward  the  sun  than 
away  from  it,  southward  if  the  sun  is  in  the  south,  eastward  during 
the  time  that  the  sun  is  in  the  east,  and  more  slowly  eastward  when 
the  sun  is  in  the  west.  The  eastward  motion  is,  in  general,  greater 
in  the  morning  and  less  in  the  afternoon.  There  is  a  slightly  greater 
central  depression  of  the  barometer  by  day  than  by  night.  The 
depression  increases  with  a  temperature  gradient  greater  than  the 
average  prevailing. 


WE  A  THER-MAPS.  1 65 

The  clear  air  and  the  colder  north-west  wind  in  the  rear  of  a 
cyclone,  combined  with  the  warmer  southerly  winds  and  cloudy  or 
rainy  weather  on  its  front,  are  the  controlling  features  that,  decide 
the  rate  and  direction  of  its  progressive  motion.  The  colder  and 
drier  the  air  is  on  the  rear  the  more  the  course  turns  toward  the 
north-east. 

The  movements  of  the  areas  of  low  pressure  in  the  United  States 
are  affected  by  matters  that  occur  far  outside  the  country.  For  the 
proper  study  of  the  atmosphere  the  limits  of  the  weather-maps  should 
be  extended  so  as  to  take  in  the  whole  northern  hemisphere. 

A  cyclonic  movement  1000  miles  in  diameter  in  North  America  is 
affected  by  the  temperatures  and  pressures  5000  miles  away  in  Europe 
and  Siberia.  Such  large  movements  in  the  atmosphere  require  a 
consideration  of  all  the  air  in  the  northern  hemisphere  as  a  unit. 

In  northern  Europe  the  direction  of  the  motion  of  a  cyclone  centre 
for  24  hours  forms  an  angle  of  60  degrees,  on  the  average,  with  a 
line  from  the  centre  of  the  low  to  the  place  of  greatest  preceding 
24-hour  temperature  rise.  The  angle  varies  in  winter  from  34  to  90 
degrees,  and  in  summer  from  30  to  no  degrees. 

TROPICAL    CYCLONES. 

Tropical  Cyclones.  —  The  cyclones  of  the  tropics  occurring  between 
30°  north  latitude  to  30°  south  form  a  special  and  distinctive  class  of 
low-pressure  areas.  They  are  not  so  great  in  extent  as  cyclones 
farther  north.  The  pressures  at  the  centres  go  very  low,  on  rare 
occasions  as  low  as  27.0  inches,  a  depth  never  known  to  be  attained 
by  cyclones  in  middle  latitudes. 

Hurricanes.  —  Exceedingly  violent  winds  occur  all  around  the  centre 
of  these  cyclones,  sometimes  as  great  as  100  miles  or  more  an  hour; 
these  winds  are  known  as  hurricanes  in  the  West  Indies.  The  region 
with  hurricane  winds  is  a  circle  or  oval-shaped  area  sometimes  as 
much  as  300  miles  in  diameter.  In  the  Bay  of  Bengal  the  severest 
are  only  60  to  80  miles  in  diameter.  The  West  India  cyclones  when 
moving  along  the  United  States  coast  produce  storm  winds  of  50 
miles  an  hour  for  a  distance  of  100  miles  inland  from  the  centres. 


1 66  METEOROLOGY. 

Storm  Wind. — A  storm  wind  is  considered  to  be  a  wind  approxi- 
mating a  velocity  of  40  miles  an  hour  on  sea  and  30  on  land. 

In  the  centre  of  a  tropical  cyclone  there  is  at  times  an  area  of  a 
few  square  miles  where  the  air  is  calm  and  the  sky  clear.  This  is 
called  by  sailors  the  "  eye "  of  the  storm.  Usually,  however,  at  the 
centre  the  cloud  is  thickest.  As  the  centre  moves  away  the  winds 
in  the  rear  of  it  rage  with  their  greatest  violence.  The  isobars  are 
usually  oval-shaped,  and  the  "eye"  is  off  towards  one  end  of  the  oval, 
in  the  direction  of  its  motion. 

Cirrus  Cloud  and  Cyclone.  —  Cirrus  clouds  and  halos  appear  on  all 
sides  around  a  tropical  cyclone.  There  is  not  the  same  difference  in 
character  of  cloud  in  rear  and  front  in  tropical  cyclones  that  there  is  in 
the  case  of  cyclones  farther  north.  Cirrus  clouds  precede  the  centre  of 
the  cyclone  500  miles.  Cirrus  clouds  in  Cuba  give  the  first  indication 
of  the  position  of  a  hurricane.  Cirro-stratus  stripes  that  appear  like 
white  and  delicate  feathers  are  the  forerunners  of  hurricanes.  Their 
form  of  divergence  corresponds  with  the  direction  of  the  centre  of  low 
pressure.  The  upper  clouds  over  the  cirrus  veil  appear  most  dense  in 
a  particular  part  of  the  horizon  where  a  whitish  arc  is  formed.  When 
the  sun  rises  or  sets  through  this  it  appears  of  an  intense  red. 

Pressure  preceding  Cyclone. — Outside  of  a  cyclone,  on  its  border, 
there  is  a  region  of  pressure  in  excess  of  the  average,  usually  30.15 
inches.  In  the  West  India  cyclone  of  September,  1875,  tnis  was 
observed  at  Havana  when  the  centre  was  1200  miles  away.  Without 
such  a  previous  high  pressure  no  storm  of  dangerous  violence  is  apt 
to  occur. 

The  ocean-swell  is  felt  to  great  distances  on  all  sides  of  a  cyclone, 
and  is  often  observed  400  miles  in  advance  of  the  centre,  and  often  48 
hours  before  the  occurrence  of  the  most  violent  winds  at  a  place.  In 
the  West  India  hurricane  of  1839  tne  swell  was  observed  at  the  Ber- 
muda Islands  when  the  centre  was  at  a  distance  of  700  miles. 

Cyclone  Wave.  —  Near  the  centre  of  a  cyclone  the  violent  winds 
produce  a  heavy  sea  all  around  the  centre.  The  waves  are  some- 
times as  high  as  40  feet,  and  the  distance  from  crest  to  crest  500 
feet.  The  wave  becomes  gradually  smaller  as  it  spreads  from  the 
origin.  The  law  of  diminution  of  crest  height  with  distance  from  the 


WE  A  THER-MAPS.  1 6/ 

centre  of  disturbance  is  according  to  the  square  root  of  the  distance. 
A  wave  that  is  40  feet  high  at  a  distance  of  4  miles  from  the  centre 
of  disturbance  will  be  4  feet  high  at  a  distance  of  400  miles. 

In  the  case  of  a  wave  such  as  a  tidal  wave,  or  the  wave  produced  by 
an  earthquake,  where  all  the  water  to  the  bottom  of  the  ocean  partakes 
of  the  motion,  the  velocity  of  the  wave-crest  is  much  greater  than  in 
the  case  of  a  wave  where  only  the  top  layer  of  the  ocean  water  is 
concerned.  The  velocity  of  a  wave  increases  with  the  depth  of  the 
water  and  the  length  of  the  wave.  For  long  waves  in  deep  water  it  is 
equal  to  the  velocity  acquired  by  a  body  in  falling  through  a  height 
equal  to  half  the  depth  of  the  water.  For  a  depth  of  one  mile,  for 
instance,  the  velocity  would  be  about  280  miles  an  hour. 

Temperature  in  Cyclone.  —  In  a  tropical  cyclone  whose  path  is  at  first 
from  east  to  west  the  temperature  is  lower  on  the  west  side  of  centre, 
the  direction  in  which  it  is  moving,  than  it  is  on  the  east  side.  The 
difference  in  temperature  is  slight,  however,  amounting  only  to  three 
or  four  degrees.  This  is  in  marked  contrast  with  the  temperature  dis- 
tribution around  a  cyclone  in  middle  latitudes,  where  it  is  much  higher 
on  the  east  than  the  west  side,  the  direction  in  which  the  centre 
usually  moves  being  toward  the  east.  The  difference  of  temperature  in 
middle-latitude  cyclones  on  the  two  sides  is  very  great,  sixty  degrees  or 
more  at  times  in  winter.  The  difference  in  temperature  on  the  two 
sides  of  the  centre  would,  therefore,  not  seem  to  be  the  cause  of  the 
motion. 

Cyclones  and  Thunder.  —  Thunder  and  lightning  are  usual  accompani- 
ments of  a  tropical  cyclone  in  its  progress.  With  those  of  the  greatest 
fury,  however,  there  is  no  lightning.  The  tremendous  violence  of  the 
cyclone  winds  destroys  plantations,  demolishes  houses,  and  wrecks 
ships  in  harbour.  In  places  subject  to  hurricanes,  when  thunder  is 
heard  people  feel  relieved,  knowing  that  the  storm  will  not  be  of  the 
most  destructive  kind.  An  ever-present  accompaniment  of  a  cyclone  is 
a  heavily  clouded,  darkened  sky,  which  pours  down  torrents  of  rain. 
Lower  down  than  the  main  body  of  cloud  there  is  often  seen  tattered 
masses  of  cloud-wrack  moving  from  the  centre  of  the  cyclone  towards 
the  edge.  The  air  is  so  filled  with  cloud  and  rain  that  it  becomes  dark  in 
the  middle  of  the  day.  On  shipboard  sea  and  sky  seem  to  blend  together. 


i68 


METEOROLOGY. 


Place  of  Origin.  —  Tropical  cyclones  originate  at  about  latitude  8°  or 
10°  on  either  side  of  the  equator.  The  centres  move  toward  the  west, 
usually  slightly  to  the  north-west  in  the  northern  hemisphere. 

In  June  and  July  the  tropical  cyclones  or  tracks  of  West  India  hurri- 
canes keep  well  to  the  south,  and  cross  the  Caribbean  Sea,  and  at 
times  also  the  island  of  Cuba,  in  a  west  by  north  direction  approxi- 
mately. From  the  beginning  of  August  to  the  end  of  September  the 


FIG.  27. 

track  is  generally  in  a  course  between  west-north-west  and  north-west. 
The  paths  all  curve  between  27°  and  33°  north  latitude.  As  the  season 
advances  the  directions  of  the  tracks  incline  more  to  the  westward,  the 
hurricanes  keep  farther  south,  and  the  paths  curve  in  lower  latitudes. 
In  the  latter  part  of  September  and  October  they  reach  Cuba  from  the 


WE  A  THER-MAPS.  1 69 

southern  part  of  the  Caribbean  Sea,  and  curve  at  the  tropic  of  Cancer 
or  below  it,  with  the  peculiarity  that  where  several  come  in  succession 
each  one  curves  in  a  lower  latitude  and  farther  west  than  the  preceding 
one.  The  intervals  in  these  cases  are  from  12  to  14  days. 

Direction  of  Motion. —  On  reaching  latitude  25°  to  30°  in  the  northern 
hemisphere,  the  direction  of  motion  changes  to  north-east.  When 
charted  on  a  map,  the  path  has  some  resemblance  to  a  parabola.  The 
farther  north  they  progress  the  more  easterly  the  motion  becomes. 
The  initial  motion  is  west,  and  in  those  of  the  northern  hemisphere, 
in  five  per  cent  of  the  cases,  it  is  a  little  south  of  west. 

In  Fig.  27  is  shown  the  track  of  the  hurricane  which  passed  over 
the  Bahama  Islands  on  the  evening  of  October  i,  1866.  The  inside 
circle  of  lowest  pressure,  710  millimeters,  is  equal  to  27.95  inches. 

Velocity.  —  The  average  velocity  of  the  West  India  cyclone  centres, 
in  the  first  part  of  their  paths,  is  about  1 5  miles  an  hour.  The  velocity 
diminishes  as  the  centre  approaches  the  turning-point  in  its  path.  At 
the  turning-point  the  centre  tarries,  sometimes  remaining  stationary 
two  or  three  days.  After  beginning  to  move  eastward,  the  velocity 
increases  as  it  moves  north-east :  the  isobars  also  then  widen  out  and 
the  winds  around  the  centre  diminish  in  velocity. 

Many  West  India  cyclones  die  out  before  reaching  the  turning-point, 
and  never  describe  a  north-east  course. 

Dangerous  Half.  —  The  dangerous  half  of  a  cyclone,  with  the  strongest 
winds  dangerous  to  ships,  is  the  northern  half  while  moving  west,  and 
the  south-eastern  half  while  moving  north-east. 

In  this  half  the  wind  tends  to  force  a  ship  directly  to  the  centre  of 
the  low  pressure.  Ships  always  manoeuvre  to  avoid  the  centre. 

Change  of  Wind  Direction.  —  During  the  progress  of  a  cyclone  at 
places  to  the  right  of  the  centre  advancing,  the  wind  changes  in  the 
tropics,  north  of  the  equator,  from  the  north  around  by  the  east  to  the 
south.  On  the  left  of  the  centre,  the  wind  changes  from  the  north 
through  the  west  to  the  south.  In  cyclones  south  of  the  equator,  these 
motions  are  reversed. 

The  wind  directions  around  the  centre  are  shown  in  Fig.  28. 

Frequency  of  Cyclones.  —  On  an  average  there  has  been  one  very 
severe  hurricane  in  the  West  Indies  every  year.  There  is  a  record 


170 


METEOROLOGY. 


for  the  past  four  hundred  years,  or  since  the  discovery  of  America. 
The  percentages  occurring  in  the  various  months  are  as  follows :  Jan- 
uary, 1.4;  February,  2.0;  March,  3.0;  April,  1.7;  May,  1.4;  June,  3.0; 
July,  1 1.6;  August,  27.2;  September,  22.5;  October,  19.4;  November, 
4.8  ;  and  December,  2.0. 

WIND  DIRECTIONS  ABOUND  A  HURRICANE. 


FIG.  28. 


List  of  Cyclones.  —  Some  of  the  most  notable  of  these  are  as  follows  :  — 

1 780.  —  Barbadoes  hurricane  ;  20,000  lives  lost. 
Jan.,       1825.  —  Fifty  vessels  driven  ashore  at  Bermuda. 
Aug.  2,  1837.  —  Island  of  St.  Thomas;  pressure  28.06  inches. 
Oct.  5,   1844.  —  Cyclone  moved  from  Cuba  to  Newfoundland  in  two  days. 
Sept.  6,  1865.  —  Guadaloupe  Island;  pressure  27.95. 
Oct.  i,    1866.  —  Nassau,  Bahama  Island;  pressure  27.72. 


WE  A  THER-MAPS.  1 7 1 

Aug.  14,  28,  1873.  —  Cyclone  turned  to  north-east  off  Cape  Hatteras;  centre  off 
shore;  destroyed  1223  vessels;  600  lives  lost. 

Aug.  15,  '  1875.  —  Cyclone  turned  to  the  north-east  at  Indianola,  Texas;  176 
lives  lost. 

Sept.  21,  1877.  —  Barbadoes  Island;  cyclone  moved  along  coast  from 
Florida  to  Massachusetts. 

Oct.  21,  24,  1878.  —  Havana,  Cuba;  cyclone  entered  United  States  on  North 
Carolina  coast. 

Aug.  16,  20,  1879.  —  Cyclone  touched  the  coast  of  North  Carolina  and  moved 
along  shore ;  at  Cape  Lookout  the  velocity  of  wind  was 
138  miles  an  hour  (uncorrected  for  error  of  anemome- 
ter.) 

Aug.  17,  18,  1880.  —  Hurricane  at  Jamaica  Island. 

Aug.  23,  28,  1881.  —  Cyclone  entered  the  United  States  on  Georgia  coast  and 
moved  north-west  to  Minnesota. 

Oct.  27,         1 88 1 .  —  Manzanilla ;  cyclone. 

Sept.  5,          1882.  —  Hurricane  at  Cienfuegos  ;  pressure  29.13  inches. 

Oct.  8,  14,     1882.  —  Hurricane;  Island  of  Cuba. 

Nov.  5,          1882.  —  Hurricane  at  Manilla;  pressure  27.35  inches. 

Aug.  19,  20,  1886.  —  Indianola,  Texas;  overwhelmed  by  sea  wave  accompany- 
ing cyclone. 

Aug.  15,         1893.  —  Severe  off  Savannah  and  Charleston. 

Bengal  Bay  Cyclones.  —  Cyclones  in  the  Bay  of  Bengal  are  of  great 
intensity.  The  centres  move  north-west  from  the  vicinity  of  the 
Andamann  Islands,  in  latitude  12°  north,  to  the  mouth  of  the  Ganges 
River  in  latitude  23°.  In  the  ten  years,  1877  to  1886,  18  occurred  in 
the  month  of  July,  17  in  August,  and  17  in  September.  These  are 
the  months  of  greatest  frequency,  Of  109  cyclones  in  ten  years,  the 
lowest  pressure,  27.13  inches,  was  that  observed  in  the  False  Point 
cyclone  of  September  19,  22,  1885.  The  velocity  of  the  centre  of  this 
cyclone  the  first  day  was  3.5  miles  an  hour,  the  second  day  8,  and  the 
third  day  14.  These  velocities  are  typical  of  all  these  cyclones,  gradually 
increasing  as  they  advance.  The  average  velocity  of  these  cyclones  is 
about  8  miles  an  hour,  but  sometimes  as  great  as  25. 

There  are  three  well-defined  regions  in  these  cyclones ;  an  outer  area 
or  ring  of  light  winds,  an  inner  area  with  very  heavy  winds,  and  an  area 
of  calm  around  the  centre.  In  passing  from  the  outer  to  the  inner 
area,  the  fall  of  pressure  is  very  great  and  sudden. 


METEOROLOGY. 

The  fall  of  pressure  in  the  case  of  the  greatest  of  these  cyclones 
begins  to  be  perceptible  at  a  place  16  hours  in  advance  of  the  lowest 
pressure,  which  may  be  200  miles  distant.  The  winds  are  even  more 
violent  than  the  hurricanes  of  the  West  India  cyclones.  The  Backer- 
gunj  cyclone  of  October  31,  1876,  was  accompanied  by  a  flood-wave 

45  feet  high,  which  covered  the  delta  of  the  Ganges  River,  drowning 
100,000  people. 

These  cyclones  are  peculiar  to  the  water.  On  reaching  land  the 
meeting  with  a  ridge  of  hills  has  the  effect  of  breaking  them  up,  indi- 
cating that  the  main  part  of  their  energy  is  in  the  lower  air,  and  prob- 
ably due  to  the  condensation  of  the  moisture  contained. 

Cyclones  do  occasionally  originate  over  the  land  in  southern  India, 
but  they  are  always  feeble  as  compared  with  those  over  the  water. 

Typhoon.  —  The  cyclones  of  the  China  Sea  are  known  as  typhoons; 

46  have  occurred  in  65  years.     This  includes,  probably,  only  the  most 
violent  ones.    They  are  very  similar  to  those  of  the  Bay  of  Bengal.    All 
this  class  of  cyclones  emanate  from  the  belt  of  permanently  low  pressure 
in  the  region  of  the  equatorial  calms. 

At  Mauritius  Island,  in  latitude  20°  south,  cyclones  of  a  similar 
nature  occur.  There  have  been  53  in  24  years  ;  the  greatest  number, 
15  each,  occurring  in  February  and  March.  The  great  storm  at  Samoa 
Island,  March  15,  1889,  was  a  tropical  cyclone  with  hurricane  winds. 

Signs  of  Cyclone.  —  In  the  tropics  the  approach  of  a  cyclone  is  very 
definitely  indicated  by  fall  of  pressure.  The  daily  range  of  pressure  in 
the  tropics  is  large  and  very  regular.  The  least  deviation  of  pressure 
variation  from  its  customary  regularity  is  an  indication  of  a  cyclone  in 
the  vicinity.  A  close  watch  of  the  change  in  the  direction  of  the  wind 
affords  a  means  of  determining  the  direction  of  the  centre.  The  aver- 
age direction  of  motion  of  cyclones  in  the  vicinity  enables  one  to  judge 
as  to  how  the  centre  will  pass  with  reference  to  the  place,  and  hence 
what  will  be  the  probable  direction  and  intensity  of  the  winds  accom- 
panying it,  judging  by  what  has  occurred  in  previous  cases.  This  is 
the  principle  on  which  predictions  of  occurrence  of  cyclone  and  high 
winds  are  made.  For  the  cyclones  in  the  vicinity  of  the  West  Indies 
the  direction  of  the  centre  from  a  ship  can  be  inferred  from  the  direc- 
tion of  the  wind  and  clouds  as  follows  :  — 


WE  A  THER-MAPS.  1 73 


Clouds  and  winds  The  centre 

move  from  will  bear 

•N.  E. 

N.E.  S.E. 

E.  S. 

S.E.  S.W. 


Clouds  and  winds  The  centre 

move  from  will  bear 

S.  W. 

S.W.  N.W. 

W.  N. 

N.W.  N.E. 


In  the  case  of  cyclones  over  the  ocean,  observations  at  only  a  single 
point  on  shipboard  or  on  an  island  are  usually  available  for  purposes  of 
prediction,  making  estimates  very  uncertain. 

Management  of  Ship  in  Cyclone.  —  When  the  pressure  falls  0.6  of  an 
inch  in  six  hours,  and  there  has  been  no  change  in  the  direction  of  the 
wind  in  that  time,  it  may  be  inferred  that  the  place  is  right  in  the  path 
of  the  centre  of  an  advancing  cyclone.  The  right  advancing  quarter 
has  the  most  violent  and  dangerous  winds  in  the  northern  hemisphere. 
In  the  case  of  a  ship,  by  sailing  or  steaming  in  a  proper  course  there  is 
always  time  to  avoid  danger.  A  ship  will  sail  away  from  the  centre  by 
keeping  the  wind  on  the  starboard  quarter  in  the  northern  hemisphere 
and  on  the  port  in  the  southern  hemisphere. 

This  applies  for  the  open  ocean,  away  from  the  coast.  In  the  region 
of  the  coast  of  the  United  States,  from  south  of  Florida  to  North  Caro- 
lina, it  is  preferable  for  a  vessel  to  be  in  the  so-called  dangerous  semi- 
circle and  move  to  the  north-east  or  east  to  get  away  from  the  centre. 
In  the  other  semicircle  the  vessel  is  in  a  precarious  position,  being 
liable  to  be  squeezed  between  the  track  of  the  hurricane  and  the  coast 
without  space  to  run.  It  is  a  bad  plan  to  allow  a  ship  to  run  with 
the  wind.  It  will  inevitably  be  carried  round  and  round  the  centre 
and  subjected  to  long-continued  strain  from  the  wind  and  waves,  with 
danger  of  final  destruction.  The  rate  of  pressure-fall  and  direction 
of  ocean-swell,  as  well  as  the  winds,  are  guides  in  estimating  the 
direction  of  a  cyclone  centre  from  a  ship.  In  most  cases  it  is  not 
possible  to  tell  the  true  direction  of  centre  from  the  one  observation  that 
can  be  made  on  shipboard.  It  is  a  good  plan  to  have  the  vessel  lie  to 
for  several  hours  in  case  of  a  great  fall  of  pressure  until  the  change  in 
direction  of  wind  can  be  accurately  ascertained.  In  the  northern 
hemisphere  strong,  squally  winds  extend  a  long  distance  from  the 
centre  towards  the  south-east,  with  very  little  change  in  the  direction 
in  many  hours.  This,  in  connection  with  a  steady  increase  of  pressure, 


174  METEOROLOGY. 

would  indicate  that  a  ship  was  in  the  south-east  quarter  of  the  cyclone, 
and  that  the  centre  was  moving  away  from  it.  The  vessel  could  then 
sail  or  steam  accordingly,  so  as  to  avoid  the  centre,  the  approximate 
direction  of  the  motion  of  centres  for  the  vicinity  being  known. 

Causes  of  Cyclones.  —  Cyclones  are  due  primarily  to  the  unequal 
heating  and  moisture,  or  cooling  and  drying,  of  the  air  over  large 
regions  of  the  earth's  surface,  disturbing  the  level  of  the  surfaces  of 
equal  density.  This  results  in  a  convectional  ascensional  movement  of 
the  lighter  air  near  the  ground  and  the  coming  down  of  heavier  air 
from  above  to  restore  the  equilibrium.  The  light  air  moves  spirally 
inward  and  upward,  and  at  a  greater  height  flows  outward  to  the  sides. 
This  flow  is  similar  to  that  of  water  from  a  basin  through  a  hole  in 
the  bottom.  The  motion  from  opposite  sides  produces  a  couple  which 
gives  rise  to  the  rotation. 

When  the  upward  convection  extends  to  a  height  at  which  the 
temperature  is  lowered  by  dynamic  cooling  below  the  temperature  of 
the  dew-point  of  the  air,  there  is  condensation  and  cloud  formation. 
When  this  occurs,  the  initial  gyratory  impulse  of  the  air  becomes  of 
secondary  consequence.  The  principal  part  in  maintaining  and  extend- 
ing the  ascending  motion  is  taken  by  the  latent  heat  set  free  by  the 
vapour.  The  reduction  of  pressure  due  to  collapse  of  the  vapour  adds 
some  to  the  action,  but  only  a  little.  The  cloud  canopy  in  the  daytime 
also  increases  the  tendency  of  the  air  to  ascend  by  transferring  the 
point  of  application  of  the  sun's  heat  from  the  ground  to  the  top  sur- 
face of  the  clouds  at  a  height  in  the  air. 

Convectional  ascending  motion  in  the  air  is  going  on  at  all  times 
during  the  day,  but  for  the  most  part  is  not  sufficient  to  carry  the  air 
high  enough  to  produce  any  great  amount  of  condensation,  sometimes 
on  account  of  the  feebleness  of  the  ascensional  force,  and  again  because 
of  the  dryness  of  the  air  requiring  ascent  to  a  very  great  height  to 
reduce  it  to  the  dew-point.  This  condition  sometimes  produces  a  dry 
cyclone  of  feeble  action,  with  cloud  formation  only,  and  no  rain. 
Cyclones  in  middle  latitudes,  though  they  do  not  always  cause  rain,  are 
almost  invariably  accompanied  by  cloud. 

The  decrease  of  pressure  in  a  cyclone  produced  by  rainfall  alone  is 
very  slight.  The  centrifugal  force  developed  by  the  gyration  and  the 


WE  A  THER-MAPS.  175 

deflecting  influence  of  the  earth's  rotation  on  the  currents  are  the  main 
causes  of  the  production  of  low  pressure  at  the  centre  of  a  cyclone. 

Where  the  deflecting  effect  of  the  earth's  rotation  is  least,  near  the 
equator,  the  winds  blow  nearly  towards  the  centre  of  the  cyclone, 
except  very  close  to  the  centre,  where  they  are  nearly  tangent  to 
the  isobars.  In  going  north,  the  winds  deviate  to  the  right  with  the 
increase  in  the  deflecting  force  of  the  earth's  rotation  due  to  increase 
of  latitude,  and  the  winds  are  more  nearly  tangential  to  the  isobars. 

Cause  of  Anticyclones  or  Highs.  —  Anticyclones  or  areas  of  high 
pressure  result  from  convectional  descending  currents  from  the  upper 
air  produced  by  increased  density  due  to  excessive  cooling  by  radiation 
from  the  upper  air  into  space.  The  first  effect  of  this  process  of 
radiation  is  to  increase  the  air  pressure  near  the  ground,  causing  an 
outflow  of  air  from  the  centre.  The  first  effect  of  cooling  is  an  increase 
in  density  of  the  air,  causing  a  contraction  and  the  inflow  of  air  from 
roundabout  to  restore  the  level  of  the  surfaces  of  equal  pressure.  The 
increased  quantity  and  weight  of  air  over  a  place  produces  the  increase 
of  pressure.  This  applies  only  to  the  great  anticyclones  of  the  middle 
and  high  latitudes.  The  belt  of  high  pressure  at  the  tropics  is  the 
result  of  air  heaped  up  by  its  expulsion  from  the  great  permanent  areas 
of  low  pressure  around  the  poles. 

The  effect  of  convection  due  to  radiation  is,  first,  to  make  the  air  of  a 
temperature  throughout  a  great  height  correspond  to  the  adiabatic  rate 
of  upward  diminution,  which  results  in  a  fall  of  temperature  at  the 
surface  of  the  earth,  and  a  rise  of  temperature  in  the  upper  air.  This 
change  of  temperature  alone  would  not  produce  any  increase  of  pres- 
sure. But  the  upper  air  quickly  cools  by  radiation  to  the  temperature 
peculiar  to  that  altitude,  the  small  thickness  of  superposed  air,  and  its 
freedom  from  moisture,  offering  very  slight  impediment  to  radiation. 
The  increased  pressure  in  a  high  area  is  solely  due  to  the  great  density 
of  the  cold  air  of  which  it  is  composed. 

In  the  northern  hemisphere,  cyclones  in  middle  latitudes  are  often 
the  result  of  a  warm  body  of  air  coming  from  a  southern  region  into  a 
cold  northern  region  favourable  to  precipitation  of  its  moisture.  A 
high  is  at  times  the  result  of  air  moving  from  the  north  to  warmer 
southern  regions. 


1 76  METEOROLOGY. 

The  appearance  of  some  of  the  greater  highs  is  coincident  with  a 
great  fall  in  temperature  over  a  wide  area  of  country.  The  cause  of  a 
high  pressure  may  be  partly  due  to  radiation  from  the  lower  air,  due  to 
the  absence  of  moisture.  High  areas  follow  after  a  wide-spread  rainfall, 
and  although  the  amount  of  moisture  in  the  air  in  the  layers  nearest 
the  earth  is  as  great  as  before  or  during  the  rain,  yet  there  is  no  doubt 
that  at  a  considerable  height  there  is  much  less  than  before,  and  the 
whole  amount  of  moisture  contained  in  the  air  is  less.  The  fact  that 
the  areas  are  much  more  extensive,  and  the  pressures  greater,  in  winter 
than  summer  harmonizes  with  this  view. 

The  circulation  of  the  air  in  a  high-pressure  area  from  right  to  left 
with  the  motion  of  the  hands  of  a  watch  increases  the  pressure  in  the 
high,  the  rotation  of  the  earth  deflecting  the  currents  to  the  right. 

That  there  is  a  great  deal  of  the  upper  air  brought  down  to  the 
surface  of  the  earth  in  a  high-pressure  area  is  shown  by  an  analysis  of 
the  air.  The  percentage  of  oxygen  contained  in  the  air  at  such  times 
is  less  than  the  average  which  is  the  characteristic  of  the  upper  air. 

In  winter  the  highs  are  of  greatest  extent  and  greatest  frequency. 
The  long  nights  of  northern  latitudes  are  favourable  to  great  cooling  by 
radiation  from  the  upper  air.  The  clear  skies  of  the  high  areas,  the  air 
containing  but  little  moisture,  are  favourable  to  the  cooling  of  the  earth 
by  radiation  during  the  night,  and  the  air  next  to  it  by  contact,  so  that 
the  lower  air  becomes  considerably  colder  than  at  a  height  above.  This 
anomaly  of  temperature  increase  with  ascent  in  highs  is  frequently  ob- 
served, more  especially  at  times  when  the  ground  is  covered  with  snow. 

It  has  been  maintained  that  the  cooling  of  the  air  in  a  high-pressure 
area  is  mainly  due  to  cooling  of  the  ground  by  radiation,  and  of  the  air 
by  contact  with  it.  But  this  can  be  true  only  of  the  lower  strata. 

Cold  in  Highs.  —  In  the  United  States  highs  rarely  last  more  than 
two  or  three  days  in  the  central  part  of  the  country.  The  lowest  tem- 
peratures in  these  occur  when  the  ground  is  covered  with  snow.  The 
heat  of  the  sun  during  the  day  goes  largely  to  melting  and  evaporating 
the  snow,  and  is  so  much  heat  disposed  of  that  would  otherwise  go  to 
raising  the  temperature  of  the  air. 

Conductivity  of  Snow.  —  Snow  by  its  low  conductivity  prevents  the 
passing  of  heat  from  the  earth  to  the  air.  Through  a  layer  of  snow  one 


WEATHER-MAPS.  177 

centimetre  thick,  with  a  difference  in  temperature  of  one  degree  centi- 
grade between  the  two  surfaces,  there  passes  through  each  square 
centimetre  of  surface  in  one  minute  of  time  0.0304  gramme-calories  of 
heat ;  through  sand  or  loam  the  amount  transmitted  is  0.205  ;  through 
iron,  9.77 ;  through  copper,  54.62. 

In  Russia  the  average  minimum  temperature  of  the  air  as  compared 
with  the  daily  mean  temperature  when  there  is  snow  on  the  ground  is 
eighteen  degrees  lower  than  when  there  is  no  snow.  In  the  United 
States  this  difference  is  probably  not  so  great. 

Remarks  on  Theory.  —  The  commonly  accepted  theory  of  the  cause 
of  cyclones  is  the  one  just  given,  which  attributes  them  mainly  to  con- 
densation of  vapour.  It  explains  many  facts  concerning  them,  but  it 
must  be  admitted  not  all.  It  is  not  very  clear  that  it  does  explain  the 
motion  of  the  lows.  Condensation  of  moisture  can  hardly  be  the  cause 
of  the  constant  procession  of  low-pressure  areas  from  north  of  North 
Dakota  to  the  Gulf  of  St.  Lawrence.  The  fact  that  the  cyclones  of  the 
Bay  of  Bengal  are  quickly  broken  up  on  reaching  land  by  even  a  low 
ridge  of  hills  would  seem  to  show  that  the  main  part  of  the  energy  in 
that  class  of  low  pressures  is  in  the  moisture  of  the  lower  air. 

It  has  been  suggested  that  cyclones  and  anticyclones  are  due  to  the 
same  causes  that  produce  the  general  circulation  of  the  air;  that  is,  to 
the  difference  of  temperature  at  the  equator  and  the  poles.  The  fact 
that  the  difference  is  greater  in  winter  than  summer,  might  explain  the 
greater  frequency  of  cyclones  in  winter.  But  as  yet  nothing  has  been 
developed  to  show  just  how  this  cause  might  act  to  produce  low  and 
high  pressures.  There  is  a  good  deal  to  indicate  that  rain  is  an  incident 
of  vertical  circulation  rather  than  a  producer  and  sustainer  of  a  moving 
area  of  low  pressure. 

The  fact  that  the  low  centre  moves  toward  the  centre  of  the  rain 
area,  seems  to  favour  the  conclusion  that  in  a  great  storm  the  conden- 
sation of  vapour  is  an  efficient  cause  which  controls  the  winds. 

There  is  evidence  that  heavy  and  extensive  rains  do  not  invariably 
precede  the  first  formation  of  depression  areas,  and  accompany  their 
expansion.  In  the  United  States  low  areas  do  not  generally  begin  with 
extensive  rainfall,  but  the  rainfall  is  a  concomitant  after  the  system  of 
circulating  winds  has  been  pretty  well  established. 


CHAPTER   VIII. 

WEATHER  PREDICTIONS. 

Basis  of  Predictions.  —  The  basis  of  weather  predictions  is  the 
approximately  known  direction  of  motion  and  rate  of  progress  of  areas 
of  low  pressure,  and  their  average  accompaniments  as  regards  rain  or 
cloud,  rise  or  fall  of  temperature,  and  the  velocity  and  direction  of  winds 
around  them.  Typical  cases  of  low-pressure  areas  for  different  parts 
of  the  country,  with  the  average  areas  of  rainfall  accompanying  and  fol- 
lowing them,  are  shown  in  the  charts  on  Plates  No.  i  to  No.  22.  These 
charts,  taken  in  connection  with  the  statistics  of  the  motions  of  the  low- 
pressure  areas  given  in  the  preceding  pages,  enable  one  to  form  some 
judgment  as  to  the  occurrence  of  rain  for  the  case  of  strongly  marked 
types  of  pressure  distribution.  Some  idea  can  be  formed  as  to  the 
direction  of  the  wind  from  the  typical  distribution  of  the  winds  around 
a  low-pressure  area,  and  of  the  strength  it  will  attain  from  the  pressure 
gradients,  the  rate  of  fall  of  pressure  which  has  already  occurred  over 
the  area  covered  by  the  low  pressure,  or  from  the  farther  fall  which 
may  be  reasonably  anticipated.  The  temperature,  fall  or  rise,  may  be 
inferred  from  the  contrast  in  the  temperature  of  the  region  on  the  front 
and  back  side  of  the  low-pressure  area. 

The  typical  wind  directions  at  the  surface  of  the  earth  around  an  area 
of  low  pressure  are  shown  in  Fig.  22. 

Any  area  of  pressure  less  than  29.92  inches  reduced  to  sea-level  with 
pressure  increasing  from  centre  is  a  low  pressure ;  any  area  above  30.0 
with  pressure  decreasing  from  centre  is  a  high  pressure. 

To  the  side  the  low  pressure  is  moving,  the  wind  has  to  change 
approximately  to  the  typical  direction  usually  before 'rain  begins.  In 
the  case  of  an  area  moving  from  the  west,  if  at  a  place  to  the  east  of 
the  centre,  the  wind  is  from  the  north-west,  it  will  change  to  north-east 
before  rain  begins.  When  the  winds  are  not  typical  the  lows  and  highs 

are  not  persistent. 

178 


WEATHER  PREDICTIONS.  179 

A  vague  irregular  area  of  low  pressure  extending  from  Colorado  east 
and  south,  if  there  is  a  high  pressure  to  the  north,  with  rounding  convex 
front  toward  the  east,  is  almost  certain  to  concentrate  in  a  definite 
regular  low  area  in  the  region  of  the  Great  Lakes. 

To  the  east  of  the  centre  of  a  low  pressure,  the  temperature  is  higher 
than  to  the  west.  As  the  low  pressure  moves  along,  the  temperature 
rises  over  the  country  to  the  east  of  it,  and  falls  to  the  west  of  it. 
Cirrus-stripe  clouds  appear  to  the  east  of  a  centre  of  low  pressure. 

All  around  a  centre  of  low  pressure,  but  principally  to  the  east  of  it, 
rain  occurs. 

The  north  and  north-west  winds  to  the  north-west  of  a  centre  of  low 
pressure,  and  the  south-west  winds  to  the  south  of  the  centre,  are  strong 
and  about  in  proportion  to  the  pressure  and  temperature  gradients  over 
the  regions. 

The  area  of  a  low  pressure  is  considered  to  include  all  the  area 
inside  of  the  last  rounding  isobar. 

To  the  west  of  a  centre  of  low  pressure,  great  areas  of  temperature 
fall  follow,  especially  if  there  is  an  area  of  high  pressure  to  the  north, 
north-west,  west,  or  south-west  of  the  centre. 

As  the  low  pressure  moves  over  the  country,  the  storm  winds,  rain 
area  and  temperature  fall  areas  follow  along  with  it. 

Estimate  of  Motion  of  Low.  —  When  a  low  has  been  visible  for  several 
days,  and  the  direction  of  its  motion  is  known,  the  best  estimate  of  its 
position  at  the  end  of  the  next  24  hours  is,  to  take  its  path  as  the 
straight  line  continuation  of  the  path  just  passed  over,  and  the  distance 
it  will  move  in  24  hours,  as  one  and  one-third  of  the  distance  it  has 
moved  in  the  past  24  hours,  in  case  the  motion  is  towards  the  north-east. 
When  the  motion  of  a  low  pressure  in  any  of  the  paths  described  is 
south-east,  its  velocity  is  usually  retarded  until  it  reaches  the  place 
where  it  turns  to  the  north-east.  At  the  turning-point  it  is  usually 
motionless  for  a  day  or  more.  When  it  turns  to  move  north-east  its 
velocity  increases  day  by  day.  When  the  centre  deepens  the  velocity 
increases. 

Motion  of  Centre  towards  Rain.  —  The  centre  of  a  low  is  apt  to 
move  towards  the  place  of  greatest  rainfall  in  the  preceding  24  hours 
to  the  east  of  it.  .'  •;  '•>  >  - 


180  METEOROLOGY. 

When  the  pressure  gradients  to  the  north-west  of  a  centre  are  heavy 
as  compared  with  those  on  other  sides  of  the  low  pressure,  the  centre 
of  the  low  will  move  in  a  line  at  right  angles  to  the  line  of  heaviest 
pressure  gradient.  This  is  nearly  north-east  usually. 

Motion  with  Reference  to  High  Dew-point. — When  the  approximate 
position  of  a  centre  of  a  low  pressure  is  located  24  hours  ahead,  as 
described,  or  by  means  of  an  average  velocity  and  average  direction, 
where  no  path  has  been  as  yet  described,  the  observations  of  dew-point 
in  the  vicinity  permit  of  locating  the  future  position  of  centre  more 
accurately.  When  there  is  a  place  within  two  or  three  hundred  miles  of 
the  centre,  with  the  temperature  of  dew-point  higher  than  any  of  the  dew- 
points  surrounding  it,  the  centre  of  the  low  pressure  after  an  interval  of 
12  hours  is  likely  to  be  located  at  the  place  of  the  highest  dew-point. 

Motion  by  Pressure  Falls. —  The  low-pressure  areas  that  appear  to 
the  north  and  move  east  have  certain  characteristic-shaped  areas  of 
pressure  fall  in  advance  of  them.  The  areas  of  1 2-hour  equal  falls  of 
pressure  are  oblong  when  charted  by  lines  of  equal  fall  through  the 
points  of  o.i,  0.2,  etc.,  of  an  inch  of  fall  and  extend  from  south-west  to 
north-east.  The  place  of  the  centre  of  low  pressure  12  hours  ahead  is 
apt  to  be  on  the  0.2  of  an  inch  pressure-fall  line  at  the  end  of  the  short 
axis  of  the  area  of  pressure  fall  farthest  from  the  centre  of  the  low- 
pressure  area. 

In  the  case  of  double  low  pressures,  one  near  the  Gulf  of  Mexico 
and  the  other  in  the  region  of  the  Great  Lakes,  the  whole  system 
moves  east  just  the  same  as  a  single  system. 

It  is  rarely  possible  to  locate  the  centre  of  a  low  pressure  more  than 
24  hours  ahead  close  enough  to  make  the  estimated  position  of  any 
use  in  forecasting  weather. 

Wind  and  Motion  of  Centre. —  From  43  cases  in  the  United  States  in 
which  the  centre  of  a  low  pressure  moved  1000  miles  in  a  day,  and  52 
cases  in  which  the  movement  in  a  day  was  244  miles,  the  velocity  of 
the  wind  on  the  different  sides  of  a  low  pressure  do  not  seem  to  have 
any  influence  on  the  rate  of  motion  of  the  centres.  Low  pressures 
moving  from  Texas  north-east  to  the  region  of  the  Great  Lakes,  or  from 
the  Gulf  of  Mexico  to  New  England,  diminish  in  pressure  at  the  centre 
about  0.3  of  an  inch,  on  the  average. 


WEATHER  PREDICTIONS.  l8l 

Variation  in  Rate  of  Pressure  Diminution. —  The  variation  in  rate  of 
pressure  decrease  is  valuable  in  indicating  whether  a  low  pressure 
moving  east  will  deepen  or  fill  up  as  it  advances. 

Places  on  sea  or  lake  coast  within  200  miles  of  a  centre  of  a  low 
pressure  will  have  storm  winds  approximating  40  miles  an  hour  when 
the  pressure  gradient  is  O.6  of  an  inch  or  more  in  500  miles  and  the 
temperature  gradient  at  least  20  degrees  in  a  distance  of  500  miles. 

A  storm  wind  is  one  with  a  velocity  approximating  40  miles  an  hour 
on  sea  and  30  on  land. 

Storm  Wind  and  Gradients. —  The  greater  the  pressure  and  tempera- 
ture gradients  the  greater  the  wind  velocity.  Hurricane  winds  extend 
no  more  than  100  miles  from  the  centre  of  the  cyclone. 

With  a  pressure  gradient  as  low  as  0.4  of  an  inch  and  the  tempera- 
ture gradient  as  high  as  40,  storm  winds  may  be  anticipated.  When 
the  pressure  diminishes  at  the  centre  the  winds  may  be  expected  to 
increase  in  velocity. 

Winds  and  Pressure  Fall. —  Storm  winds  need  not  be  anticipated  at 
a  place  unless  there  occurs  to  the  west  or  to  the  south  of  it  large  areas 
of  12-hour  pressure  fall  of  0.3  of  an  inch  or  more  extending  over  an 
area  at  least  100,000  square  miles  in  extent. 

An  area  of  high  pressure  moving  to  the  east  from  the  Lakes  on  the 
coast  gives  north-east  storm  winds  from  New  Jersey  to  Nova  Scotia. 
The  strength  of  the  winds  depends  on  the  pressure  and  temperature 
gradients ;  a  pressure  has  usually  to  be  at  least  30. 5  to  give  a  storm 
wind. 

High-pressure  areas  of  30.3  in  Manitoba  with  considerable  tempera- 
ture gradient  will  sometimes  give  storm  winds  from  the  north  on  Lake 
Michigan. 

A  given  pressure  gradient  in  summer  produces  a  greater  wind 
velocity  than  the  same  gradient  in  winter. 

The  high  pressures  that  advance  to  the  east  from  the  interior  of  the 
country  have  storm  winds  on  their  south-east  front  on  the  sea-coast. 

Rain  and  Low. —  As  a  rule,  the  appearance  of  a  low  pressure  and  the 
occurrence  of  rain  are  simultaneous  over  the  greater  part  of  the  area 
inside  of  the  last  rounding  isobar  of  the  low  pressure.  Rain  is  likely  to 
occur  in  24  hours  over  all  the  area  covered  by  the  low  pressure  and  in 


lS2  METEOROLOGY. 

the  area  of  country  over  which  it  passes  as  it  moves  along  and  as 
far  as  300  miles  at  least  from  the  centre  to  the  east  of  it.  The  great 
downpours  of  rain  accompanying  hurricanes  never  extend  more  than  200 
miles  from  the  centre. 

Quantity  of  Rain. —  No  definite  estimate  can  be  made  of  the  depth  of 
rainfall  that  will  occur  with  a  low-pressure  area.  In  the  case  of  a  low 
pressure  from  the  Gulf  of  Mexico,  it  may  be  taken  for  granted  at  least 
half  an  inch  will  fall  at  any  place  over  which  the  low  pressure  will  pass, 
and  possibly  two  inches  or  more.  This  is  especially  the  case  with  a 
high-pressure  area  to  the  north-east  of  the  centre  of  the  low  pressure. 
This  combination  of  pressure  gradient  gives  rise  to  north-east  winds 
extending  from  New  England  to  Georgia,  and  constitutes  the  well- 
known  north-east  storm  along  the  Atlantic  coast.  This,  with 
other  conditions,  tends  to  show  that  rainfall  is  merely  an  incident 
of  the  circulation  of  which  the  isobars  are  the  result.  If  the  air 
contains  a  great  deal  of  moisture,  or  the  circulation  extends  to 
a  sufficient  height,  rain  follows;  if  but  little  moisture,  there  may  be 
only  formation  of  clouds.  Rain  occurring  to  the  west  of  a  centre  of 
low  pressure  will  show  there  is  enough  moisture  to  produce  rain ;  and 
when  this  occurs,  taken  in  connection  with  the  typical  isobars,  rain  to 
the  east  of  the  centre  is  tolerably  sure  to  follow  as  the  low  pressure 
moves  along. 

Low-pressure  areas  that  move  from  the  Gulf  of  Mexico  are  always 
accompanied  by  great  rainfalls. 

When  the  paths  of  extensive  low-pressure  areas  cross  the  southern 
part  of  the  United  States  in  winter,  periods  of  cold  weather  will  follow 
over  the  eastern  part  of  the  United  States  to  the  north  of  the  path,  and 
about  a  hundred  miles  south  of  the  path. 

High  and  Rain.  —  On  the  south-east  front  of  an  inland  high-pressure 
area,  without  any  definite  accompanying  low  pressure,  light  rains  occur 
at  times,  usually  less  than  o.  I  of  an  inch. 

After  long-continued  rains  in  the  region  from  Virginia  to  Maine,  the 
weather  will  clear  within  24  hours  when  the  pressure  to  the  south-west 
from  Alabama  and  Tennessee  to  Texas  begins  to  rise. 


WEATHER  PREDICTIONS.  183 

Low  pressures  that  first  appear  north  of  North  Dakota,  and  move 
east,  may  have  little  or  no  rainfall,  especially  if  the  depression  is  only 
of  slight  depth.  Long-continued  wind  from  the  ocean,  24  to  36  hours, 
will  produce  rain  along  the  coast  at  times  without  any  low  pressure 
accompanying  it. 

Duration  of  Rain.  —  With  low  pressures  from  the  Gulf  of  Mexico, 
cloudy  weather,  with  occasionally  rain,  will  last  at  least  I  day,  some- 
times 3  days.  Great  rainfalls  are  mostly  over  in  8  hours,  and  only 
rarely  last  uninterruptedly  for  24  hours. 

Rain  Frequency  in  Low.  —  Dividing  the  area  of  a  low  pressure  into 
four  regions,  the  centre,  the  first,  the  second,  and  the  third  rings,  the 
number  of  times  of  the  occurrence  of  rain  out  of  ten  cases  in  each 
region,  are  respectively  8,  6,  4,  and  3. 

Dew-point  and  Rain.  — The  occurrence  of  rain  at  a  place  is  closely 
related  to  the  temperature  of  dew-point  and  its  rise.  Along  the  coast 
of  the  Gulf  of  Mexico,  and  along  the  south  Atlantic  coast,  a  dew-point 
temperature  as  high  as  78°  at  8  A.M.,  is  sure  to  be  followed  by  rain 
within  24  hours.  In  the  same  region,  a  rise  in  the  temperature  of  the 
dew-point  of  only  three  degrees,  is  sure  to  be  followed  by  rain.  In 
January,  in  the  region  of  the  Great  Lakes,  a  rise  sometimes  as  great  as 
eighteen  degrees  in  the  temperature  of  the  dew-point  occurs  before 
there  is  rain.  The  amount  of  increase  and  the  temperature  of  dew- 
point  which  invariably  precede  rain  is  different  in  different  sections  of 
the  country,  and  varies  slightly  in  the  same  section  at  different  times 
of  the  year.  Charts  representing  these  critical  values  of  dew-point 
and  dew-point  change  for  various  parts  of  the  country  are  of  value  in 
estimating  the  probable  occurrence  of  rain  in  connection  with  the 
other  indications  given  by  cloudiness,  pressure,  and  pressure  change. 

Frequency  and  Quantity  of  Rain.  —  The  occurrence  of  rain,  and  more 
especially  the  quantity  of  rain,  depends  on  the  amount  of  vapour  in  the 
air,  as  well  as  on  pressure  gradients.  When  the  air  is  nearly  saturated 
in  summer  time,  very  slight  pressure  gradients,  even  such  as  are  hardly 
perceptible  on  the  weather-maps,  are  associated  with  great  downpours 
of  rain. 

The  pressure  gradient  in  summer  is,  in  fact,  scarcely  of  any  use  in 
indicating  a  coming  rain.  Moist  air  and  a  barometric  pressure  only 


I84 


METEOROLOGY. 


one-tenth,  or  even  only  a  few  hundredths  of  an  inch  below  the  average 
of  30.0  inches,  may  give  numerous  scattered  rains  over  a  wide  area, 
which  are  usually  classified  and  predicted  as  local  rains. 

Secondary  Low  Pressures.  —  Secondary  depressions  consist  of  a  kink 
or  loop  in  one  or  more  of  the  isobars  of  a  low.  The  secondary  is  a 
low  pressure  on  a  low  pressure.  Rain  is  very  apt  to  occur  in  and 
around  secondary  low  pressures,  and  especially  thunderstorms  in  sum- 
mer time.  They  occur  mostly  to  the  south  or  east  of  a  centre  of  low 
pressure.  In  Fig.  29,  thunderstorms  are  very  apt  to  occur  with  sec- 

SECONDARY  DEPRESSION. 
29.7 


FIG.  29. 

ondary  low  pressures.  Thunderstorms  are  most  apt  to  occur  when 
the  temperature  is  considerably  above  the  average  for  the  time  of  the 
year ;  they  are  most  apt  to  occur  when  the  pressure  in  its  transition 
from  a  maximum  to  a  minimum  is  approaching  the  mean  pressure 
peculiar  to  the  place. 

What  has  been  said  with  regard  to  rain  holds  true  for  snow  with  the 
temperature  of  the  air  below  32°.  Snow  sometimes  occurs  with  the 
temperature  as  high  as  40°.  In  winter,  when  the  low  centre  is  below 
Washington  City,  it  gives  snow  instead  of  rain. 

Remarks  on  Predictions.  —  Predictions  of  rain,  with  the  aid  of  weather- 
maps,  as  indicated,  are  on  the  whole  far  from  satisfactory,  and  it  is 
doubtful  whether  they  are  of  any  particular  value  in  many  cases.  Very 
generally  the  rain  begins  with  the  appearance  of  a  low  pressure  over  a 
great  area  of  country,  both  being  simultaneous,  and  the  most  that  can 
be  said  in  the  way  of  prediction  is,  that  the  rain  will  continue  for  some 


WEATHER  PREDICTIONS.  185 

indefinite  time  at  places  where  it  is  already  raining.  As  much  can  be 
said  on  general  principles,  on  the  ground  of  probability,  without  the  aid 
of  a  weather-map.  Rain  is,  moreover,  preceded  by  a  cloudy  sky,  so 
that  one  can  often  form  as  good  a  judgment  regarding  the  coming 
of  rain,  by  means  of  local  signs,  as  with  the  aid  of  the  most  com- 
plete weather-map.  This  is  more  especially  true  in  summer.  Tre- 
mendous downpours  occur  occasionally  without  any  marked  peculiarity 
of  pressure  distribution.  At  two  places,  not  more  than  fifty  miles  apart, 
there  may  be  a  fall  of  4.0  inches  of  rain  at  one,  and  none  at  the  other. 
In  winter,  however,  with  strongly  marked  low  pressures  and  high  pres- 
sures of  certain  types,  it  is  possible  to  successfully  foretell  for  a  con- 
siderable time  ahead  many  of  the  great  rain  and  snow  storms. 

The  uncertainty  of  a  weather  prediction  is  greater  the  greater  the 
interval  of  time  for  which  it  is  made  after  the  epoch  of  the  weather- 
map  on  which  it  is  based.  For  this  reason  the  official  government  pre- 
dictions of  the  Weather  Bureau  printed  in  the  newspapers  are  not  of 
much  value,  being  based  on  the  weather-map  of  the  preceding  day  or 
even  earlier.  For  the  purpose  of  quick  announcements  signal  flags  are 
displayed  at  the  various  Weather  Bureau  offices  throughout  the  country 
indicative  of  the  weather  expected.  These  flags  are  shown  on  page  186. 
It  is  difficult  to  remember  the  meanings  of  all  the  various  combinations  of 
flags  in  use.  A  system  for  memorizing  the  flags  that  has  been  found 
convenient  for  the  wind-signal  flags  is  as  follows  :  The  flag,  as  displayed 
on  the  page  can  be  likened  to  a  map  ;  the  top  will  be  north  ;  when  the 
flag  is  white  the  first  letter  of  the  word  "white"  is  indicative  of  west. 
A  white  pennant  above  the  red  flag  then  means  a  wind  from  the  north- 
west ;  if  below,  from  the  south-west.  The  red  indicates  a  direction 
opposite  to  the  white,  or  from  the  east.  A  red  pennant  above  a  flag 
therefore  indicates  a  north-east  wind,  and  below  it  a  south-east. 


METEOROLOGY. 


I87 


No.  1. 


Clear  or 
Fair. 


RAIN   AND  TEMPERATURE   SIGNALS. 


No    2. 


No-  3. 


- 


Local 
Rains. 


No.  4. 


Temperature 
Signal. 


No-  5. 


Cold 
Wave. 


No.  i  indicates  clear  or  fair  weather;  when  displayed  alone  that  temperature  will  remain  sta- 
tionary. No.  2  indicates  rain  or  snow  ;  when  displayed  alone  that  temperature  will  remain  station- 
ary. No.  3  indicates  that  local  rains  or  showers  will  occur  and  that  the  rainfall  will  not  be  general  ; 
displayed  alone  that  temperature  will  remain  stationary.  No.  4  always  refers  to  temperature.  When 
placed  above  Nos.  i,  2,  and  3,  it  indicates  warmer  weather;  when  placed  below,  colder  weather. 
No.  5  indicates  the  approach  of  a  sudden  and  decided  fall  in  the  temperature.  When  No.  5  is  dis- 
played, No.  4  is  always  omitted. 


|a 


Violent 
Storm. 


STORM,    CAUTIONARY,    AND   INFORMATION    SIGNALS. 


Light 
Storm. 


Easterly 
Winds. 


Westerly 
Winds. 


Information 
Signal. 


Light 
Storm, 
N.  W. 
Winds. 


Light 
Storm, 
S.W. 
Winds. 


Light 
Storm, 
N.  E. 
Winds. 


Light 
Storm, 
S.  E. 
Winds. 


The  Information  Signal  denotes  a  storm  covering  a  limited  area,  dangerous  only  to  vessels  intend- 
ing to  sail  for  certain  points.     Information  can  be  had  by  applying  to  the  local  Observer. 

Night  Signals.  —A  red  light  indicates  easterly  winds ;  a  white  light  above,  westerly  winds. 


WEATHER  PREDICTIONS.  189 

Spectroscope  and  Rain. — The  indications  of  the  twinkling  of  stars, 
supposed  to  be  due  to  a  great  deal  of  moisture  in  the  air,  also  red  sun- 
sets, and  at  times  the  transparency  of  the  air,  are  of  no  value  in  rain 
prediction.  When  light  from  the  sky  is  examined  with  a  spectroscope, 
which  is  a  prism,  dispersing  the  light,  separating  it  into  its  component 
colours,  dark  bands  or  lines  more  or  less  pronounced  are  seen  in  the 
spectrum  which  depend  on  the  amount  of  moisture  in  the  air.  These 
lines  known  as  the  rainband,  are  of  no  value  in  indicating  coming  rain. 

In  regard  to  the  prediction  of  storm  winds,  there  are  only  few  cases 
where  serviceable  predictions  can  be  made,  and  these  are  for  the  cases  of 
very  deep  and  regular  low-pressure  areas.  As  a  rule,  the  approach  to 
a  storm  wind  velocity  is  very  gradual  through  all  the  lower  velocities, 
from  1 5  and  20  miles  an  hour,  up  to  40.  The  continual  increase  of  the 
wind  is  warning  enough  to  sailors  to  seek  shelter  when  possible,  espe- 
cially on  the  lakes,  and  warnings  are  in  only  rare  instances  of  practical 
value. 


COLD    WAVES. 

Cold  Waves.  —  As  a  low  area  of  pressure  moves  over  a  region,  there 
follows  west  of  it  an  area  of  temperature  fall.  As  a  high  area  of  pres- 
sure moves  along,  falls  of  temperature  occur  to  the  south-east  of  it. 
When  there  occurs  a  low  pressure  and  with  it  a  high-pressure  area  to 
the  north,  north-west,  west,  or  south-west  of  it,  the  falls  in  temperature 
are  apt  to  be  large  and  extend  over  great  areas. 

A  fall  of  temperature  of  20  degrees  in  24  hours,  extending  over  an 
area  of  at  least  50,000  square  miles,  and  the  temperature  in  any  part  of 
the  area  going  at  least  as  low  as  36°  is  called  a  "cold  wave."  The 
number  of  cold  waves  that  have  occurred  in  the  United  States  in 


190 


METEOROLOGY. 


the  ten  years,  1880  to  1890,  arranged  according  to  the  months  and 
the  extent  of  country  enclosed  by  the  2O-degree  temperature-fall  line, 
is  as  follows  :  — 

NUMBER  AND  AREA  OF  COLD  WAVES,   1880  TO  1890. 


AREA. 

OCT. 

Nov. 

DEC. 

JAN. 

FEB. 

IVlAR. 

YEAR. 

50,000  to     100,000  square  miles  .     . 

21 

32 

31 

13 

23 

30 

1$° 

100,000  to       200,000       "             "         .      . 

13 

14 

30 

27 

26 

24 

134 

200,000  to     300,000      "          "       .     . 

2 

12 

21 

25 

'9 

13 

92 

300,000  to     400,000      "          "       .     . 

I 

10 

16 

29 

24 

II 

91 

400,000  to     500,000     "          "       .     . 

I 

8 

14 

16 

13 

12 

64 

500,000  to     600,000     "          "       .     . 

5 

5 

13 

10 

I 

34 

600,000  to     700,000     "          "       .     . 

I 

I 

9 

6 

2 

»9 

700,000  to     800,000     "         "       .     . 

• 

3 

4 

5 

I 

!3 

800,000  to     900,000      "          "       .     . 

I 

5 

3 

2 

I 

12 

900,000  to  1,000,000     "         "      .     . 

2 

I 

i 

2 

6 

1,000,000  to  1,100,000      "          "       .     . 

i 

2 

3 

1,100,000  to  1,200,000     "         "       .     . 

2 

I 

3 

Extent  of  Fall.  —  The  extent  of  these  cold  waves  is  very  great  at 
times.  The  greatest  in  ten  years  occurred  January  17,  1882.  The  area 
enclosed  by  the  lo-degree  temperature-fall  line  was  2,900,000  square 
miles;  that  by  the  2O-degree,  1,100,000;  that  by  the  3O-degree,  539,000; 
that  by  the  4O-degree,  14,000.  The  greatest  fall  at  the  centre,  at  Den- 
ison,  Tex.,  was  44  degrees. 

The  greatest  fall  in  temperature  at  the  centre  of  a  cold  wave  varies 
in  different  cases.  There  have  been  2  cases  in  ten  years  in  which 
the  greatest  falls  at  the  centre  in  24  hours  were  over  60  degrees ;  16 
between  50  and  60 ;  77  between  40  and  50  degrees ;  262  between  30 
and  40 ;  and  264  between  20  and  30  degrees. 

The  weather-map  of  December  31,  1884,  and  the  cold  wave  that  fol- 
lowed the  next  day,  are  shown  on  the  adjoining  page.  On  the  upper 
map  the  red  lines  are  the  isobars,  or  lines  of  equal  pressure ;  the 
blue  lines  are  the  isotherms,  or  lines  of  equal  temperature.  On  the 
lower  map  the  red  lines  join  the  points  of  equal  24-hour  temperature 
fall ;  the  2O-degree-fall  area  is  within  the  10,  the  30  within  the  20,  and 
the  40  within  the  30. 


WEATHER  PREDICTIONS.  IQI 

Cold- Wave  Cone.  —  The  areas  enclosed  by  the  temperature-fall  lines 
are  approximately  elliptical.  As  a  rule,  they  agree  much  better  with  this 
shape  than  in  the  example  given.  Regular  ellipses  will  in  most  cases 
represent  the  falls  of  temperature  with  errors  no  greater  than  6  degrees. 

The  fall  of  temperature  in  a  cold  wave  may  be  considered  graphi- 
cally as  a  cone.  The  greatest  fall  in  temperature  at  the  centre  of  the 
cold  wave  is  the  altitude  of  the  cone  ;  the  10,  20,  30,  and  40  degree  tem- 
perature-fall lines  are  intersections  of  planes  with  the  cone. 

Intensity  and  Dimensions  of  Cold  Wave.  —  The  dimension  of  a  cold 
wave  is  taken  as  proportional  to  the  cubic  contents  of  the  cone  or  the 
area  of  the  base  multiplied  by  one-third  of  the  altitude.  The  unit  of  area 
is  taken  as  100,000  square  miles  and  the  unit  of  altitude  as  10  degrees  of 
temperature  fall.  To  find  the  extent  of  a  cold  wave,  the  areas  enclosed 
by  the  temperature-fall  lines  are  measured  with  a  planimeter.  The  con- 
tents is  computed  as  a  rough  cone.  No  allowance  is  made  in  comput- 
ing the  extent  for  any  area  covered  by  a  fall  of  temperature  less  than 
10  degrees. 

The  area  of  temperature  fall  between  a  lo-degree  fall  and  the  line  of 
no  fall  cannot,  as  a  rule,  be  measured  accurately,  owing  to  the  indefi- 
niteness  of  the  lines  of  zero-temperature  change.  The  extent  of  tem- 
perature fall  in  a  cold  wave,  as  computed,  is  then  more  accurately  a  cone 
of  fall  greater  than  10  degrees,  surmounting  a  cylinder  of  lo-degree  fall. 
The  extent  of  different  cold  waves  measured  in  this  way  varies  from  3.0 
to  60.0. 

Excess  and  Deficency  of  Pressure.  —  The  deficiency  of  pressure  in  a 
low-pressure  area  and  the  excess  of  pressure  in  a  high-pressure  area  may 
be  estimated  as  cones,  in  the  same  way  as  the  extent  of  a  cold  wave,  from 
the  measurements  of  the  areas  between  the  isobars  on  the  weather-map. 
The  unit  of  area  is  taken  as  100,000  square  miles,  and  the  unit  of  excess 
or  deficiency  of  pressure  as  one  inch.  The  extent  of  a  high  or  low 
pressure  area  on  this  basis  is  sometimes  less  than  unity  and  at  times  as 
great  as  ten. 

The  weather  and  temperature-fall  maps  show  that  the  extent  of  a 
cold  wave  depends  on  the  extent  of  the  low  pressure  preceding  it,  the 
extent  of  the  high  pressure  coming  after  it,  and  the  density  of  the  iso- 
therms in  the  region  from  the  centre  of  the  low  pressure  towards  the  high. 


192 


METEOROLOGY. 


Fall  of  Temperature  and  Gradients.  —  The  relation  between  the 
greatest  falls  of  temperature  in  cold  waves  and  the  pressure  and  tem- 
perature gradients  to  the  north-west  of  the  centre  of  low  pressure 
are  as  follows  for  the  average  of  a  number  of  cases  :  — 


NUMBER  OF  CASES. 

GREATEST  FALL  OF 
TEMPERATURE. 

INCREASE  OF  PRESSURE 
IN  500  MILES. 

DECREASE  OF  TEMPERA- 
TURE IN  500  MILES. 

16 

53.6  degrees 

0.66  inches 

55  degrees 

60 

43-3       " 

0.56      " 

47       " 

60 

33-5       " 

0.59      '< 

44      " 

60 

24.0       " 

0.46      « 

34       " 

60 

14-3       " 

0.40      " 

23       « 

Predictions  of  cold  waves  are  made  from  the  weather-maps,  when  a 
low  and  a  high  pressure  are  shown,  and  the  pressure  and  temperature 
gradients  are  sufficient  to  produce  a  fall  considerably  greater  than  20 
degrees. 

Place  of  Greatest  Fall.  —  The  place  of  greatest  temperature  fall  in  a 
prospective  cold  wave  can  be  taken  at  the  point  of  highest  temperature 
on  the  weather-map,  within  a  distance  of  100  miles  of  the  centre  of  the 
low  pressure.  In  about  eight-tenths  of  all  the  cases  of  cold  waves,  this 
will  be  at  a  point  south  of  the  centre  of  the  low  pressure. 

In  .the  case  of  a  high  pressure  to  the  south-west  of  a  low  pressure, 
the  place  of  greatest  temperature  fall  in  many  instances  is  farther  to  the 
south  of  the  low  pressure  than  100  miles.  In  cases  of  this  kind,  the 
region  of  greatest  temperature  fall  is  apt  to  be  a  ridge  extending  from 
south-west  to  north-east  rather  than  a  point.  The  selection  of  the  point 
of  actual  greatest  fall  in  such  cases  cannot  usually  be  made  with  great 
accuracy,  but  the  point  selected  will  at  least  be  at  a  point  of  great  fall. 

In  some  cases  the  greatest  fall  of  temperature  is  to  the  west,  to  the 
north,  or  to  the  east  of  the  centre  of  the  low  pressure,  usually  about 
the  place  where  the  temperature  diminishes  most  rapidly  toward  the 
north-west  and  at  the  place  of  highest  temperature  in  the  region  about 
the  centre  of  low  pressure. 

Fall  of  Temperature.  —  At  the  place  of  greatest  prospective  tempera- 
ture fall  in  a  cold  wave,  the  temperature  after  the  fall  is  the  weighted 


WEATHER  PREDICTIONS. 


193 


mean  temperature  of  the  sections  along  a  line  drawn  from  the  place  of 
greatest  fall,  through  the  region  of  greatest  diminution  of  temperature 
toward  the  north-west.  The  weight  to  be  given  to  the  temperature  of 
each  section  is  directly  proportional  to  the  length  of  the  section,  and 
inversely  as  the  distance  of  the  centre  of  the  section  from  the  place  of 
greatest  fall.  The  various  sections  of  the  line  are  the  lengths  included 
between  the  isotherms.  The  temperature  of  a  section  is  the  mean  tem- 
perature of  its  bounding  isotherms.  The  temperature  of  the  first  part 
of  the  line  from  the  place  of  greatest  fall  fora  distance  of  250  miles  is 
taken  with  a  weight  of  unity.  This  rule  applies  only  in  case  the  pres- 
sure gradient  is  in  excess  of  0.4  of  an  inch  in  500  miles.  The  compu- 
tation of  the  temperature  fall  at  the  place  of  greatest  fall,  for  the 
weather-map  of  February  n,  1887,  is  given  below. 

FEBRUARY  n,   1887. 


DISTANCES  BETWEEN 
ISOTHERMS. 

MEAN 
TEMPERATURE 
OF  SECTION. 

LENGTH 
OF  SECTION. 

CENTRE  OF 

SECTION  FROM 
POINT  OF 
GREATEST  FALL. 

ASSIGNED 
WEIGHT. 

MEAN 
TEMPERATURE 
x  WEIGHT. 

Degrees.     Miles. 

Miles. 

Miles. 

—  30  to  60,    i  loo  . 











—  20  to  60,        800   . 

-25° 

300 

950 

0.32 

-    8.0 

10  to  60,      690  . 

-15° 

no 

750 

0.15 

—     2.2 

o  to  60,      500  . 

-    5 

190 

590 

0.32 

-    1.6 

10  to  60,      380  . 

5 

120 

440 

0.27 

+  1.4 

20  tO  60,         270    . 

15 

110 

360 

0.31 

+  4.7 

(20-63) 

42 





I.OO 

42.0 

Sums                . 

2.77 

16  3 

Mean  temperature 
Fall  of  temperature 


-47-7 


The  temperature  on  February  n,  1887,  at  Lamar,  Mo.,  the  place  of 
greatest  fall, .was  63°.  The  next  day  it  was  9°.  The  computed  fall  is 
48  degrees.  The  fall  that  actually  occurred  was  54  degrees. 

Position  of  Long  Diameter  of  Fall  Area.  —  The  long  diameter  of  a  20 
degree  temperature-fall  area  usually  lies  in  a  direction  from  south-west 
to  north-east,  or  in  the  direction  of  the  long  axis  of  the  low  pressure 
area.  This  corresponds  to  the  average  direction  of  the  isotherms  to 


194  METEOROLOGY. 

the  west  and  south-west  of  the  centre  of  the  low.  The  long  diameter 
of  a  temperature-fall  area  always  extends  in  the  direction  of  the  open 
isobars  of  a  low.  The  2O-degree  temperature-fall  areas  on  two  suc- 
cessive days  rarely  overlap.  This  never  occurs  except  in  cases  where 
one  of  the  areas  exceeds  400,000  square  miles,  and  even  then  the  over- 
lap is  only  throughout  a  narrow  strip  of  country  scarcely  more  than 
50  miles  in  width. 

The  limits  of  the  2O-degree  temperature-fall  area  to  the  north-east,  or 
to  the  south-west,  can  be  fixed  somewhat  nearly  by  comparison  of  the 
temperatures  on  weather-map  with  the  lowest  temperatures  for  the  time 
of  the  year  occurring  in  those  particular  regions.  There  is  usually  no 
2O-degree  fall  to  be  anticipated,  when  it  would  carry  the  temperature 
below  the  average  lowest  temperature  for  the  month,  as  derived  from  a 
number  of  years,  unless  the  conditions  are  very  strong,  for  instance, 
proximity  to  the  place  of  greatest  fall. 

Of  211  cold  waves,  the  above  rule  for  computing  the  amount  of  tem- 
perature fall  at  the  place  of  greatest  fall  gave,  in  14  cases,  the  actual 
fall  of  temperature  with  an  error  of  less  than  one  degree ;  in  36  cases 
±  I  ;  in  24  cases  ±  2  ;  in  22  cases  ±  3  ;  in  14  cases  ±  4 ;  in  18  cases  ±  5  ; 
in  19  cases  ±  6  ;  in  10  cases  ±  7 ;  in  15  cases  ±  8 ;  in  8  cases  ±9 ;  in  5 
cases  ±  10;  in  4  cases  ±  n  ;  in  3  cases  ±  12 ;  in  7  cases  ±  13  ;  in  3 
cases  ±  14;  in  i  case  ±  15  ;  in  4  cases  ±  16;  in  I  case  ±  17;  and  in  2 
cases  ±  1 8.  The  errors  are  commonly  greater  in  large  temperature  falls 
than  small  ones.  The  probable  error  for  falls  between  20  and  30  de- 
grees is  ±  2.5  ;  between  50  and  60  it  is  ±  5.0. 

Frost  Predictions.  —  At  periods  of  the  year  when  frost  may  be  antici- 
pated harmful  to  growing  crops,  with  the  temperature  in  the  evening 
about  50°,  a  rough  estimate  of  how  low  the  temperature  will  go  during 
the  night  can  be  made  from  the  readings  of  a  wet  and  dry  bulb  ther- 
mometer;  the  difference  between  the  two  readings  multiplied  by  2.5, 
and  subtracted  from  the  dry-bulb  reading  will  give  the  temperature 
approximately.  This  condition  of  the  air  occurs  only  when  the  sky  is 
clear  and  the  air  pressure  prevailing  is  30. 1  inches  or  more.  When  it  is 
a  matter  of  only  a  few  degrees  to  prevent  a  frost,  protection  may  be 
afforded  in  preventing  radiation  by  covering  tender  plants  with  a  light 
cloth  or  layers  of  straw,  or  even  by  producing  a  cloud  of  smoke. 


WEATHER  PREDICTIONS.  195 

LONG-TIME    PREDICTIONS. 

Long-time  predictions  of  any  value  —  that  is,  for  several  days, 
weeks,  months,  or  a  year  —  are  impossible.  Some  of  the  most  inter- 
esting features  of  the  weather,  and  of  the  greatest  value  to 
know,  such  as  droughts,  heated  terms,  and  wet  periods,  cannot  be 
foreseen. 

The  only  approach  to  success  that  has  been  made  in  this  direction 
is  for  the  rainfall  in  India.  Some  success  it  seems  is  attained  in  long- 
time predictions  of  the  amount  of  rain  for  several  months  ahead  in 
the  middle  Ganges  valley  of  northern  India.  Unusually  heavy  and 
late  snowfall  to  the  north-west  of  the  Himalaya  Mountains  exercises 
a  retarding  influence  on  the  summer  monsoon  by  keeping  down  the 
temperature  of  the  region,  which  is  one  of  the  goals  toward  which 
the  monsoon  blows.  The  pressure  gradients  in  the  upper  air  producing 
westerly  winds  are  intensified  by  this,  and,  in  consequence,  the  easterly 
winds  which  prevail  in  India  as  far  south  as  the  Ganges  valley,  and 
ordinarily  extend  up  to  the  level  of  the  lower  cirrus  clouds  in  the  rainy 
season,  are  greatly  restricted  in  depth,  in  horizontal  extent,  and  also 
in  duration.  The  result  is,  a  diminution  of  rainfall  to  a  disastrous 
degree,  causing  the  failure  of  crops  and  sometimes  famine.  The 
observed  depth  of  snowfall  in  the  Himalayas  thus  permits  of  forming 
some  idea  of  the  quantity  of  rainfall  to  be  expected  in  the  succeeding 
rainy  season  in  the  lower  Ganges  valley.  In  five  great  famines  between 
1782  and  1877,  a  year  of  very  sparse  rainfall  in  southern  India  was 
succeeded  the  next  year  by  a  sparse  rainfall  in  northern  India.  Coinci- 
dence is  too  improbable  to  be  fortuitous,  but  the  cause  of  the  connection 
is  not  known. 

The  dry  periods  and  heated  terms  that  occur  at  times  in  the  United 
States  are  associated  and  dependent  upon  the  passage  of  low-pressure 
areas  moving  from  west  to  east  across  the  northern  part  of  the  country, 
inducing  southerly  winds  over  a  wide  stretch  of  country  to  the  south  of 
them  without  any  accompanying  rain.  There  is  no  known  cause  why 
the  low-pressure  areas  occur  at  such  times  only  over  the  northern  part 
of  the  country.  All  attempts  to  show  a  seasonal  occurrence  of  these 
phenomena,  or  to  prove  that  a  deficiency  or  excess  of  rainfall  or  temper- 


196  METEOROLOGY. 

ature  in  one  season  is  made  up  in  the  next  following  season  or  some 
other  season,  have  proved  futile. 

The  heated  terms  always  occur  in  connection  with  droughts.  When 
the  rainfall  for  two  months  in  succession  is  only  one-half  the  average 
amount,  the  result  is  a  drought ;  the  intensity  of  the  drought  is  greater 
the  less  the  rainfall  and  the  longer  the  dry  period  continues.  The 
excessive  heats  of  August,  1876,  from  Maine  to  Virginia  and  west  to 
Ohio,  were  coincident  with  a  rainfall  of  only  one-fourth  to  one-half  the 
amount  for  August  over  the  region,  averaging  only  about  one  inch. 
Temperatures  of  near  100°  and  over  occurred  on  several  successive 
days  from  Jacksonville  and  Montgomery  northward  to  Pittsburg  and 
New  York.  In  connection  with  the  unparalleled  heated  term  of  July 
to  September,  1881,  which  affected  the  entire  country  east  of  the 
Mississippi,  there  occurred  the  most  extensive,  prolonged,  and  disas- 
trous drought  ever  known  in  the  United  States.  In  July  and  August 
the  drought  was  also  severe  in  Kansas  and  Arkansas.  During  August 
less  than  one-eighth  of  the  usual  amount  of  rain  fell  in  the  Ohio  valley, 
and  less  than  one-third  along  the  Atlantic  coast. 

In  1886  a  severe  and  prolonged  drought  prevailed  in  part  of  Dakota 
and  Minnesota  from  about  the  middle  of  June  to  the  end  of  October. 
In  July  it  included  Nebraska,  Iowa,  and  parts  of  Wisconsin  and  Kansas. 
During  this  time  there  was  only  2.85  inches  of  rainfall  from  June  26  to 
September  16,  a  period  of  81  days. 

In  some  parts  of  the  west  the  variations  of  rainfall  from  year  to  year 
are  very  variable.  Some  years  there  is  rain  enough  for  the  growth  of 
crops,  and  others  are  so  dry  that  any  crop  is  impossible. 

The  climate  of  the  whole  interior  of  Australia  is  excessively  variable 
as  to  rainfall,  rendering  the  larger  part  of  it  uninhabitable  at  most  times. 

In  the  matter  of  the  prediction  of  the  beginning,  continuance,  and 
cessation  of  droughts  and  heated  terms,  nothing  can  as  yet  be  done. 
It  is,  however,  not  entirely  hopeless  that  it  may  be  possible  some  time 
in  the  future  to  predict  these  occurrences.  When  the  general  circu- 
lation of  the  air  in  the  northern  hemisphere  is  understood  in  relation 
to  the  distribution  of  the  air  pressure,  it  may  be  possible  to  predict 
the  weather  with  an  approach  to  accuracy  for  some  considerable  time 
ahead. 


WEATHER  PREDICTIONS.  197 

At  present  very  important  points  have  been  developed  in  regard  to 
weather  in  Europe,  extending  over  two  or  three  weeks,  in  relation  to 
the  positions  of  the  permanent  low-pressure  areas  over  the  north  Atlan- 
tic Ocean  and  the  permanent  high  pressure  that  extends  from  Bermuda 
to  the  Azores  and  the  high  pressure  of  Siberia  in  the  winter  time.  The 
shifting  boundaries  of  these  areas  are  associated  with  very  profound 
modifications  of  the  weather  over  a  great  part  of  Europe. 

There  are  three  principal  places  of  low  pressure  in  the  northern 
hemisphere,  —  one  south-west  of  Iceland,  one  near  Davis  Strait,  and  one 
in  the  Polar  Sea.  Around  these  the  circulation  takes  place,  and  as  one 
or  the  other  is  deeper,  it  has  an  important  bearing  on  the  weather  pre- 
vailing in  western  Europe. 

When  the  high-pressure  area  of  the  south  Atlantic  extends  very  far 
east,  the  south-west  winds  from  it  bring  ocean  air  over  Europe,  and 
a  milder  winter  than  in  the  case  where  the  high  pressure  moves  off 
north-east  and  access  of  ocean  air  is  cut  off. 

In  Siberia  in  winter  there  is  ordinarily  a  high-pressure  area  which 
extends  unbroken  for  a  distance  of  3800  miles  from  north  to  south  and 
4900  miles  from  east  to  west.  When  there  is  a  division  in  this,  which 
sometimes  occurs  along  the  drainage  basin  of  the  Obi  River,  the  western 
half  of  the  high-pressure  area  is  farther  west  than  usual,  and  produces 
or  is  associated  with  severe  winters  in  Russia  and  sometimes  Germany. 

The  idea  prevails  very  extensively,  that  weather  is  dependent  very 
largely  on  mere  local  influences,  but  it  is  not  so.  Proximity  to  sea  and 
mountains  are  important  and  have  their  influence,  but  for  the  most 
part  lakes,  valleys,  and  forests  have  scarcely  any  perceptible  effect. 
Wurtemberg,  Bavaria,  and  Switzerland,  where  the  characteristics  of  the 
country  are  so  different,  are  nevertheless  dry  or  rainy  at  the  same  time, 
and  this  is  because  weather  is  the  result  of  the  general  circulation  of 
the  air. 


CHAPTER   IX. 

RIVERS  AND  FLOODS. 

Kinds  of  Floods.  —  Floods  are  produced  as  follows :  By  sea  waves 
accompanying  earthquakes,  by  storm  waves  accompanying  cyclones,  by 
seiche  waves  overflowing  lake  shores,  by  the  slow  rise  of  lake  surfaces, 
by  the  bursting  of  glacier  lakes,  by  the  giving  way  of  ice  dams,  earth 
embankments,  or  levees,  by  the  choking  of  river  channels  due  to  the 
luxuriant  growth  of  vegetation,  by  avalanches  and  landslides,  by  the 
overflow  of  river  banks  due  to  great  rainfall  or  a  rapid  rate  of  rainfall, 
and  by  a  river  cutting  its  banks  and  changing  its  course.  Overflows 
due  to  great  rainfall  are  mainly  treated  of  here. 

Earthquake  Floods.  — A  wave  from  the  ocean  produced  by  earth- 
quake shock  sometimes  causes  great  destruction.  The  greatest  re- 
corded example  of  an  earthquake  wave  was  that  at  Lisbon,  Portugal, 
November  i,  1755.  More  than  4000  persons  were  swallowed  by  the 
yawning  earth,  and  60,000  were  drowned  by  the  wave  80  feet  high 
which  advanced  from  the  sea.  The  shock  of  this  earthquake  was  felt 
over  an  extent  of  12,000,000  square  miles  of  the  earth's  surface,  or 
four  times  the  area  of  the  United  States. 

Cylone  Floods.  —  Floods  from  storm  waves  are  sometimes  very 
disastrous.  The  storm  wave  accompanying  the  Backergunge  cyclone 
inundated  the  delta  of  the  Ganges,  destroying  more  than  100,000  per- 
sons ;  it  was  40  feet  in  height.  Many  disastrous  floods  have  occurred 
in  Holland  due  to  storms  breaking  the  dykes  which  protect  a  great 
part  of  the  country  from  being  covered  by  the  sea.  A  great  catas- 
trophe of  this  kind  occurred  in  the  year  1230,  and  a  still  greater  one 
November  I  and  2,  1570,  when  40,000  lives  were  lost. 

Seiches.  —  A  seiche  is  a  sudden  increase  in  the  height  of  the  surface 
of  a  lake.  Seiches  occur  on  the  Great  Lakes  of  the  United  States 
and  also  on  the  lakes  of  Switzerland.  They  are  sometimes  called 

198 


RIVERS  AND  FLOODS.  199 

"swashes."  The  increased  height  of  water  surface  lasts  but  a  short 
time,  and  is  sometimes  as  great  as  six  feet.  The  cause  of  a  seiche 
wave  is  not  known.  They  often  occur  in  calm  weather.  A  notable 
case  of  seiche  occurred  at  Cleveland,  Ohio,  at  6.20  A.M.,  June  23,  1882, 
which  was  associated  with  the  funnel-shaped  cloud  of  a  tornado  moving 
over  Lake  Erie.  The  wave  came  with  a  change  of  wind  from  the  south 
to  the  north-west. 

Lake  Floods.  —  Floods  from  steady  rise  of  lake  surface,  due  to  long- 
continued  great  rainfall  over  a  drainage  basin,  are  not  of  any  very  great 
importance.  Floods  of  this  kind  occur  on  the  shores  of  some  Asiatic 
lakes.  The  Great  Lakes  of  the  United  States  and  Canada  are  not 
subject  to  such  overflows. 

Reservoir  Floods.  —  The  giving  way  of  reservoir  dams  often  pro- 
duces disastrous  floods.  A  notable  instance  of  this  kind  was  the  break- 
ing of  the  dam  of  the  South  Fork  reservoir  in  western  Pennsylvania, 
on  June  i,  1889,  which  caused  the  loss  of  2500  lives  at  Johnstown. 
The  reservoir  was  3^  miles  long,  i  mile  to  i^  wide,  and  100  feet  deep 
in  some  places.  All  this  water  was  precipitated  on  Johnstown,  18 
miles  below,  by  the  giving  way  of  a  dam  1000  feet  wide,  which  caused 
the  stupendous  fatality. 

Ice-Dam  Floods.  —  Great  floods  occur  when  temporary  lakes  are 
released  by  the  breaking  of  ice  dams  caused  by  the  advance  of  a 
glacier  blocking  a  stream.  The  flood  from  a  jam  of  floating  ice  is  not 
of  much  importance  except  in  a  flat  country.  The  Missouri  River 
flood  of  1 88 1  was  largely  an  ice-jam  flood,  caused  by  a  series  of  ice 
dams.  The  peculiar  case  of  an  advancing  glacier  blocking  a  stream 
occurs  frequently  in  Switzerland.  In  1818  in  the  valley  of  Bagnes  south 
of  Martigny,  the  winter  being  very  severe,  the  Gintroz  glacier  advanced 
and  blocked  the  Dranse  River,  making  a  lake  a  mile  long,  700  feet 
wide,  and  200  deep.  The  ice  was  pierced  artificially  and  the  water 
about  half  drained  off,  when  the  barrier  broke,  causing  a  great  flood 
below.  No  lives  were  lost,  as  the  break  had  been  anticipated  for  a 
long  time.  Nearly  the  same  thing  occurred  in  1595,  but  with  great 
loss  of  life. 

Indus  Floods.  —  In  the  region  of  the  north-west  Himalayas,  the 
upper  tributaries  of  the  Indus  are  often  held  back  in  this  way,  and 


200  METEOROLOGY. 

finally  precipitated  as  floods.  A  notable  case  of  this  kind  occurred 
in  1835,  when  whole  villages  were  swept  away  for  a  distance  of  300 
miles  along  the  stream.  The  wave  crest  moved  with  a  velocity  of  25 
miles  an  hour. 

The  greatest  example  of  the  bursting  of  a  glacier  lake  known  was 
that  on  the  Shyok  River,  a  branch  of  the  Indus,  which  occurred  in 
1841,  on  the  south  slope  of  the  Karakorum  Mountains,  caused  by  the 
Biafo  glacier.  The  length  of  the  lake  was  12  miles,  its  width  2.5 
miles,  and  its  depth  200  feet.  When  the  barrier  broke,  the  flood  came 
down  the  Indus,  and  at  Attock,  300  miles  distant,  part  of  the  Sikh 
army  under  Golab  Singh,  encamped  on  its  bank,  was  destroyed. 

Floods  from  Dams  caused  by  Vegetation.  —  Floods  caused  by  luxu- 
riant vegetation  in  tropical  rivers  are  of  no  very  great  consequence. 
Floods  of  this  kind  occur  on  the  Nile  above  Sobat,  in  the  Lualaba,  a 
tributary  of  the  Congo  in  Africa,  and  on  the  Parana  in  South  America. 

Rainfall  Floods.  —  Floods  that  arise  from  excessive  rainfall  gorging 
a  river  channel  can  be  foretold  along  the  lower  courses  of  rivers  from 
the  progressive  character  of  flood  waves  when  the  water  stages  along 
the  upper  courses  are  known  and  observations  of  the  stages  of  water 
in  previous  floods  have  been  made  along  the  river  course. 

Floods  by  Channel  Changes.  —  No  predictions  can  be  made  of  earth- 
quake or  storm-wave  floods,  nor  of  river  floods,  where  they  arise  from 
a  river  changing  its  course.  These  latter  are  the  most  tremendous 
and  disastrous  of  all  floods.  No  region  of  the  world  is  subject  to 
such  great  floods  of  this  kind  as  the  parts  of  China  traversed  by  the 
Hwangho  River.  The  place  where  this  river  empties  into  the  sea  has 
varied  in  historical  times  a  distance  of  350  miles  along  the  sea-coast. 
At  every  change  enormous  loss  of  life  occurs.  In  the  fifteen  years, 
from  1851  to  1866,  it  has  been  estimated  that  the  changes  in  the  bed 
of  the  Hwangho  have  caused  by  drowning  and  by  destruction  of  crops 
the  loss  of  30,000,000  to  40,000,000  lives. 

RAINFALL    FLOODS. 

Rivers.  —  Rivers  and  lakes  are  products  of  rainfall.  Of  the  rain 
that  falls,  a  great  part  sinks  into  the  earth,  a  small  part  enters  into 
the  structure  of  plants  and  animals,  and  part  runs  directly  into  the 


RIVERS  AND  FLOODS.  2OI 

rivers  and  lakes  and  to  the  sea.  Of  the  rain  that  sinks  into  the  earth 
a  part  is  evaporated  into  the  air  and  part  runs  out  of  the  earth  as 
springs  at  lower  levels  than  it  entered.  About  one-fourth  of  all  the 
rainfall  at  a  place  attains  a  depth  of  three  feet  under  the  surface  of 
the  ground ;  the  remainder  is  evaporated  from  the  top  layers  of  the 
earth  or  runs  out  in  springs. 

Inland  Drainage  Areas.  —  In  landlocked  areas  the  rivers  run  to 
the  lowest  part  of  the  drainage  area  and  form  a  lake.  A  region  of 
this  kind  is  called  an  "  inland  drainage  area."  All  the  rainfall  in  such 
an  area  is  evaporated.  Rivers  are  slightly  charged  with  salt  and  other 
minerals  from  the  ground  through  which  they  run.  The  constant  con- 
centration of  the  minerals  by  evaporation  of  water  from  the  surface  of 
the  lake  renders  it  salty.  Great  Salt  Lake,  Utah,  contains  17  per  cent 
of  salt  and  other  minerals.  Of  lakes  of  this  kind,  the  Caspian  Sea, 
with  an  area  of  180,000  square  miles,  is  the  largest  in  the  world. 

Amount  of  Inland  Areas. — The  amount  of  land  surface  of  the 
earth  consisting  of  closed  basins  or  inland  drainage  areas  is  :  in  Aus- 
tralia, 52  per  cent  of  the  whole  country;  in  Africa,  31  per  cent;  in 
Europe  and  Asia,  28  per  cent,  mainly  in  Asia ;  in  South  America,  7.2 
per  cent ;  and  in  North  America,  3.2  per  cent. 

Lake  Rivers.  —  In  drainage  areas,  almost  landlocked,  where  the 
rainfall  exceeds  the  evaporation,  there  is  a  rise  of  its  lake  waters 
until  the  lowest  point  of  the  enclosing  ridge  is  reached,  when  it  begins 
to  overflow  as  a  river  to  the  sea.  The  stages  of  water  in  rivers  flow- 
ing from  large  lakes  is  nearly  uniform  all  the  year  round.  The  St. 
Lawrence  River,  which  rises  in  Lake  Ontario,  is  subject  to  no  more 
than  three-feet  fluctuation  along  any  part  of  its  course.  The  connect- 
ing rivers  of  the  Great  Lakes,  the  Niagara,  the  Detroit,  and  the  St. 
Mary's  oscillate  only  a  few  feet,  and  even  a  great  part  of  this  oscillation 
is  due  to  the  effect  of  wind  on  the  water  surface,  and  the  level  is  very 
nearly  constant. 

Total  Rainfall.  —  The  total  rainfall  over  the  land  surface  of  the 
earth  in  a  year  is  estimated  to  be  28,000  cubic  miles  of  water,  and 
the  amount  flowing  yearly  into  the  sea  about  7000  cubic  miles. 
Twenty-two  hundred  cubic  miles  of  the  rainfall  falls  on  inland  drainage 
areas. 


2O2  ME  TE  OR  0  LOG  Y. 

The  periodicity  of  rainfall  over  a  country  is  reflected  in  the  oscilla- 
tion of  its  rivers,  sometimes  dry  and  at  times  overflowing  their  banks. 

RIVER    COURSES. 

Thalweg.  —  Slope  of  the  ground  determines  the  way  in  which 
water  runs.  It  collects  in  the  valley  bottoms,  which  are  called  the 
"thalweg,"  and  moves  along  like  a  ball  rolling  down  an  inclined  plane. 

The  amount  of  rainfall  that  sinks  into  the  earth  depends  on  the 
permeability  of  the  soil,  on  the  slope  of  the  ground,  and  on  the  rate  of 
rainfall.  The  degree  of  permeability  of  ground  is  highly  important 
in  estimating  river  flow  from  rainfall.  All  ground  is  more  or  less  per- 
meable to  water,  but  soils  are  classified  broadly  as  permeable  or  imper- 
meable. The  ground  above  granite  or  lower  cretaceous  bed  rock  is 
impermeable.  The  impermeable  prevails  largely  in  hilly  and  moun- 
tainous regions. 

Characteristics  of  Impermeable  Ground.  —  A  marked  feature  of 
impermeable  ground  is  the  great  frequency  of  streams.  In  a  valley 
over  granite  bed  rock,  there  will  scarcely  be  ten  acres  without  a  rivulet 
or  a  ravine  at  the  bottom  of  it.  In  ground  classified  as  permeable, 
streams  are  infrequent :  there  is  neither  rivulet  nor  ravine  in  the 
vicinity  of  the  thalweg,  and  cultivation  of  the  soil  is  carried  on  to  the 
very  bottom.  When  at  great  intervals  a  creek  does  appear,  the  flow 
of  water  is  often  found  to  diminish  as  the  stream  progresses,  due  to 
the  water  sinking  into  the  earth.  This  difference  in  number  of  streams 
over  permeable  and  impermeable  ground  is  sufficiently  marked  to  be 
noticeable  on  the  maps  of  two  such  regions. 

In  permeable  ground  the  thalweg  is  almost  free  of  water  even  in  the 
heaviest  showers.  In  impermeable  ground  the  water  runs  along  the 
smallest  furrows.  When  it  rains,  every  furrow  becomes  a  rivulet,  every 
fold  of  the  ground  a  torrent,  and  the  bottom  of  every  valley  a  creek  or 
river.  The  rapid  motion  of  the  water  carries  soil  with  it,  and  the 
streams  of  such  a  region  are  heavily  loaded  with  silt.  In  some  cases 
silt  comes  from  the  rapid  current  cutting  the  banks  of  a  stream ;  so 
that  silting  is  not  always  an  indication  of  impermeable  soil. 

Rivers  and  Rainfall.  —  The  amount  of  water  contained  in  permeable 
ground  to  the  bed  rock  is  about  one-third  of  the  depth  of  the  soil.  The 


RIVERS  AND  FLOODS.  2O3 

amount  of  rainfall  that  runs  off  in  rivers  varies  widely  at  different  times 
and  in  different  places.  This  quantity  is  called  the  "run-off."  In 
Australia  the  Darling  River  drains,  on  the  average,  only  1.5  per  cent  of 
the  rainfall  over  its  drainage  basin.  In  different  years  it  varies  from 
o.i  per  cent  to  6  per  cent.  In  Europe  the  river  drainage  is  about 
one-fourth  of  the  rainfall ;  in  the  Ohio  valley  it  is  one-fourth ;  in  the 
Missouri  valley  only  one-eighth.  When  ground  is  frozen  the  amount  of 
rainfall  drained  may  be  as  much  as  90  per  cent. 

Rise  of  Rivers. — The  rivers  in  impermeable  ground  rise  with  great 
rapidity  at  first  and  then  more  slowly.  They  fall  as  rapidly  as  they 
rise.  In  permeable  ground  the  rise  and  fall  of  rivers  is  always  slow. 

River  Basin. — The  basin  of  a  river  is  the  tract  of  country  which  it 
drains.  This  is  also  called  the  drainage  area,  the  catchment  basin,  and 
the  watershed. 

The  boundaries  separating  drainage  basins  are  called  water-partings 
or  divides.  These  vary  from  the  sharp  ridge  of  a  mountain  range  to  a 
slight  roll  in  the  plain  imperceptible  to  the  eye. 

Where  the  ridge  is  worn  down  so  that  two  adjoining  basins  connect, 
the  water-parting  is  called  "  quaquaversal "  as  in  the  Cassiquiare  River 
which  connects  the  Orinoco  and  Rio  Negro. 

The  vertical  height  of  a  river  surface  in  feet  above  low  water  at  a 
place  is  called  the  "stage  of  the  river."  In  some  localities  the  channel 
depth  is  called  the  stage. 

As  the  water  of  a  river  moves,  it  is  joined  by  water  from  other 
rivers ;  these  rivers  are  called  tributaries  and  affluents  ;  the  place  of 
meeting  with  the  main  stream  is  the  confluence. 

River  Slope. — The  slight  inclination  of  a  river  surface,  in  the  direc- 
tion in  which  the  water  flows,  is  called  the  "slope."  The  greater  the 
volume  of  a  river,  usually,  the  less  its  slope.  In  small  rivers  and  moun- 
tain streams  the  slope  is  in  some  cases  as  much  as  4  to  7  feet  per  mile. 
In  great  rivers,  like  the  Mississippi,  it  is  not  more  than  0.23  of  a  foot 
in  a  mile,  on  the  average.  For  a  rising  stage  at  a  place,  the  slope 
increases  ;  with  a  falling  stage  it  diminishes. 

The  velocity  of  water  in,  a  river  is  greater  the  greater  its  slope,  the 
greater  the  area  of  its  cross-section,  and  the  greater  the  depth  of  the 
water. 


204  ME  TE  OR  O  LOG  Y. 

There  is  not  a  close  connection  between  river  slope  and  velocity, 
such  that  the  velocity  can  be  computed  from  the  slope.  All  the 
attempts  to  derive  formulae  for  velocity  in  natural  channels  from 
observed  slope  have  proved  futile.  This  is  in  large  measure  due  to  the 
very  variable  cross-section  of  a  river  even  at  points  very  close  together. 
In  fact  it  is  sometimes  observed  that  the  inertia  of  the  water  will  some- 
times carry  it  against  a  slope.  The  velocity  is  approximately  propor- 
tional to  the  square  root  of  the  slope  and  also  to  the  square  root  of  the 
mean  hydraulic  depth. 

Wetted  Perimeter. — The  length  of  a  line  from  one  margin  of  a 
river  to  the  other,  measured  along  the  bottom,  is  called  the  "wetted 
perimeter." 

The  area  of  a  cross-section  divided  by  the  wetted  perimeter  is  called 
the  "mean  hydraulic  depth." 

Great  slope  and  shallow  water  produce  a  riffle. 

An  abrupt  lowering  of  a  river  surface  produces  a  "  falls "  or  a 
cataract. 

Regimen.  —  The  characteristics  of  a  river  as  to  its  customary  rise 
and  fall,  greatest  and  least  discharge  of  water,  character  of  slope  and 
area  of  cross-section  at  different  stages  and  in  different  parts  of  its 
course  constitute  its  regimen. 

Flood  Line.  —  The  danger  line  or  flood  line  of  a  river  is  an  arbitra- 
rily selected  stage,  farther  rise  beyond  which  it  is  presumed  may  cause 
damage. 

Low-water  stages  of  rivers  are  mainly  of  interest  to  boatmen.  High 
stages  are  a  matter  of  great  concern  to  the  population  along  river 
courses,  in  localities  where  the  banks  are  apt  to  be  overflowed  at  a  high 
stage  and  the  country  inundated. 

Different  Classes  of  Flood  Streams.  —  The  way  in  which  high  water 
occurs  in  rivers  is  very  various  in  different  parts  of  the  world.  In  the 
rivers  of  Siberia  and  British  America  that  flow  into  the  Arctic  Ocean, 
high  water  is  the  result  of  snow  melting  in  spring  in  the  lowlands  up  to 
a  height  of  about  3000  feet  above  sea  level  and  flowing  north  over  ice 
and  frozen  ground.  The  Obi  and  Yenisei  in  Siberia  are  of  this  type. 
The  rivers  flow  from  south  to  north.  Along  the  upper  courses  the 
snow  melts  first,  and  as  the  water  flows  down  it  is  met  by  the  water 


RIVERS  AND  FLOODS.  2O5 

from  the  snow  farther  north,  melting  later,  and  the  consequence  is 
excessively  high  water  along  the  lower  courses  of  the  rivers. 

In  some  Siberian  rivers  high  water  occurs  only  in  mid-winter  and  is 
due  to  the  choking  of  the  channel  by  the  formation  of  ice  during  the 
excessive  cold  of  60°  or  lower  that  often  prevails  for  a  considerable 
length  of  time.  This  does  not  produce  a  flood. 

Along  the  Mackenzie  River,  floods  are  caused  by  the  head  waters 
flowing  over  the  lower  river  while  frozen,  covering  great  areas  of  land. 

While  not  more  than  25  per  cent  of  the  amount  of  rainfall  over  a 
drainage  area  runs  into  the  rivers,  the  amount  of  melted  snow  that  runs 
off  in  the  case  of  frozen  ground  may  be  as  much  as  72  per  cent. 
(Measured  at  Moscow,  Russia.) 

Rivers  from  Snow.  —  There  is  a  class  of  rivers  that  derive  their 
high  waters  from  snow  melting  in  the  mountains.  The  melting  is  a 
slow  process,  and  high  water  occurs  very  gradually  and  with  great 
regularity.  The  rivers  that  take  their  rise  in  the  mountains  of  central 
Asia  are  of  this  type,  —  the  Indus  of  India,  for  example. 

Tropical  Rivers.  —  Another  class  of  rivers  receive  water  from  rain 
only,  and  have  high  water  in  summer.  These  are  tropical  rivers,  and 
high  water  depends  on  the  rains  of  the  belt  of  calms  and  the  monsoon. 
The  greatest  rivers  of  the  world  belong  to  this  class,  the  Amazon,  the 
Congo,  the  Ganges,  the  Brahmaputra,  the  Yangtse-Kiang,  and  the 
Nile.  Rivers  near  the  equator  have  two  high  waters  in  a  year,  corre- 
sponding to  the  two  periods  of  rain  when  the  sun  passes  the  zenith 
in  moving  from  south  to  north,  and  again  in  moving  from  north  to 
south. 

Nile.  —  The  Nile,  a  tropical  river,  is  the  greatest  irrigating  stream 
in  the  world.  A  rise  less  than  the  average  causes  great  distress  among 
the  inhabitants  of  Egypt,  from  failure  of  crops  for  want  of  irrigating 
water.  A  rise  much  beyond  the  average  is  destructive  from  the  inun- 
dation it  causes.  The  lowest  high  water  in  30  years  at  Cairo,  Egypt, 
was  14.4  feet  in  1864;  the  highest,  26.2  feet  in  1878,  when  the  whole 
delta  to  Alexandria  was  flooded.  The  lowest  water  occurs  in  June,  and 
the  highest  in  the  latter  part  of  September.  The  rise  is  due  to  the 
monsoon  rains  in  Abyssina  between  latitude  5°  and  15°  north.  The 
great  fertility  of  lower  Egypt  is  due  to  the  alluvium  deposited  by 


206  METEOROLOGY. 

the  flood  waters  of  the  Nile.  The  water  is  loaded  with  black  earth, 
and  contains  traces  of  nitre. 

Yangtse-Kiang.  —  The  Yangtse-Kiang  rises  regularly  in  mid- 
summer about  50  feet  at  Hwangho  near  the  middle  of  its  course. 
Sometimes  the  rise  is  as  great  as  56  feet,  and  a  vast  area  of  country 
is  flooded.  The  floods  in  this  river  are  especially  disastrous,  occurring 
at  a  time  of  the  year  when  the  crops  are  growing.  The  rains  that  cause 
the  rises  continue  from  May  to  July.  The  flood  of  1849  lasted  from 
July  to  December.  The  Chinese  do  not  consider  that  any  great  part 
of  the  flood  water  is  from  the  melting  of  snow  in  the  mountains. 
Records  have  been  kept  of  all  the  floods  that  have  occurred  in  this 
river,  extending  back  to  922  B.C.  The  summer  rises  of  the  Yangtse- 
Kiang,  the  Amur,  and  Hwangho  show  that  the  monsoon  winds  pene- 
trate Asia  to  a  considerable  distance  inland. 

Amazon.  —  The  Amazon  River,  of  South  America,  receives  most  of 
its  water  from  rainfall,  a  very  little  coming  from  melting  snow  in  the 
Andes  Mountains.  It  has  the  largest  drainage  basin  of  any  river  in  the 
world.  In  some  parts  of  the  basin  the  yearly  rainfall  is  280  inches. 
There  is,  however,  no  very  great  variation  in  the  stages  of  the  Amazon. 
When  the  northern  tributaries  are  in  flood,  those  from  the  south  are  at 
a  low  stage,  and  when  the  southern  tributaries  are  in  flood  those  from 
the  north  are  low.  The  Orinoco  has  greater  fluctuations  than  any 
other  river  in  the  world,  the  average  annual  variation  being  70  feet. 

Temperate-Zone  Rivers.  —  Another  class  of  rivers  receive  their 
water  from  rain  and  have  high  water  in  winter  or  spring.  The  Missis- 
sippi and  Ohio  belong  to  this  class,  and  also  the  rivers  of  Europe,  the 
Rhine,  the  Seine,  and  the  Elbe.  In  this  class  the  heaviest  rains  pro- 
ducing high  water  occur  in  winter  and  spring,  and  are  specially  efficient 
in  filling  the  rivers  more,  because  they  occur  at  a  time  when  the 
evaporation  is  small  or  the  soil  frozen  and  more  of  the  rain  goes  into 
the  rivers. 

In  the  summer,  after  long-continued  dry  weather,  the  top  layer  of 
the  ground  becomes  so  dry  that  it  takes  about  three  inches  of  rain  to 
soak  it,  before  much  of  the  water  will  run  off.  Floods  do  sometimes 
occur  in  these  rivers  in  summer.  The  main  cause  of  floods  is  great 
quantity  and  rapid  rate  of  rainfall.  A  slow  rainfall  of  two  inches 


RIVERS  AND  FLOODS.  2O? 

extending  over  three  days  may  produce  only  a  very  slight  rise  in  a 
river,  while  the  same  amount  in  two  hours  may  produce  a  very  great 
one.  A'  rapid  rainfall  forms  a  water  surface  over  the  ground,  which 
promotes  a  rapid  transfer  of  a  great  part  of  it  to  the  streams. 

Snow-water  Rivers.  —  Some  rivers  receive  a  great  part  of  their  water 
from  rain,  but  high  water  is  due  to  additional  water  coming  from  melted 
snow.  Where  the  water  from  melted  snow  is  even  only  as  great  as  one- 
fifth  of  the  amount  of  rainfall,  yet  so  much  of  it  goes  to  the  rivers 
when  melted,  the  ground  being  frozen,  that,  added  to  the  ordinary  rain- 
fall, it  produces  very  high  water.  The  rivers  of  New  England,  and 
sometimes  the  Ohio  River,  belong  to  this  class  of  streams. 

Sub-tropical  Rivers.  —  Another  class  of  rivers  receive  their  waters 
from  rain  and  are  much  higher  in  winter  than  summer,  even  going  nearly 
dry  in  summer.  This  class  of  rivers  is  peculiar  to  southern  Europe  and 
the  high  waters  are  related  to  the  sub-tropical  rains,  that  is,  little  or  no 
rainfall  in  summer.  The  Arno  and  Tiber  in  Italy,  the  Loire,  Rhone, 
and  Garonne  in  France,  the  Tagus,  Mancanarez,  and  Guadalquivir  in 
Spain  are  of  this  class.  The  rivers  of  California  and  Oregon  are 
of  this  class  also,  but  receive  some  water  from  melting  snow  in  the 
mountains. 

Along  the  lower  course  of  the  Rhone  is  a  district  subject  to  great 
floods.  The  loss  of  life  and  property  was  very  great  in  the  flood  of 
November  10  to  20,  1840,  and  in  the  overflow  of  1860  even  greater. 

Intermittent  Streams. — There  is  a  class  of  intermittent  streams 
which,  on  account  of  dryness  of  climate  and  irregularity  of  rain,  flow 
only  when  rain  falls.  Temporary  creeks  and  rivers  form  and  vanish 
soon  after  the  rain  is  over. 

Gulches.  —  The  beds  of  such  streams  are  known  as  ravines  ;  in  Cali- 
fornia they  are  called  "gulches."  Some  rivers  flow  for  a  distance  and 
are  lost  in  the  ground,  to  reappear  farther  on  or  not  at  all.  These  are 
sometimes  known  as  "  lost  rivers." 

Desert  Streams. — The  great  deserts  like  the  Sahara,  the  Kalahari, 
and  the  Gobi  are  regions  without  rivers,  though  there  is  considerable 
rainfall.  In  the  desert  of  Sahara,  however,  there  are  some  continuous 
streams  in  the  mountains.  Sometimes  such  regions  are  traversed  by 
rivers  that  bring  water  from  a  long  distance.  The  Rio  Grande,  the 


208  ME  TE  OROLOG  Y. 

Nile,  and  the  Indus  are  types  of  this  class,  also  the  Colorado  from  lati- 
tude 36°  to  the  mouth  of  the  Gila  river  in  Arizona. 

Glacier  Rivers.  —  In  the  regions  of  perpetual  snow  and  ice  the  place 
of  rivers  is  supplied  by  glaciers  which  carry  to  the  sea  or  warmer  regions 
the  surplus  of  snow  which  is  not  evaporated.  The  melting  of  glaciers 
is  the  origin  of  some  rivers.  Such  rivers  have  a  milky  appearance  from 
the  great  amount  of  pulverized  rock  carried. 

Ice-jam  Floods.  —  Ice  is  carried  along  in  streams  at  the  time  of 
breaking-up  of  the  frozen  rivers  in  the  spring.  At  times,  narrow  parts 
of  the  channel,  or  sections  obstructed  by  bridge  piers,  become  gorged 
and  produce  floods  back  of  them  ;  when  the  barrier  yields,  floods  ensue 
below  them.  A  flood  of  this  kind  occurred  in  the  Maumee  River  at 
Toledo,  Ohio,  February  n  to  14,  1881  ;  and  one  at  Washington  City, 
February  12,  1881,  by  an  ice  gorge  at  Long  Bridge. 

Snow  Floods.  —  With  great  depth  of  snow  on  frozen  ground  great 
floods  are  occasionally  produced  in  rivers  not  otherwise  subject  to  over- 
flow. About  once  in  a  century  the  river  Somme  in  France  overflows 
from  this  cause.  The  flood  of  1658,  which  was  preceded  by  six  weeks 
of  excessively  cold  weather,  was  due  entirely  to  melting  snow  which  lay 
on  the  ground  to  the  depth  of  six  feet.  Preceding  the  great  flood  of 
1740  the  conditions  were  similar. 

Weather  and  River  Rises.  —  Very  little  has  as  yet  been  discovered 
in  the  way  of  dependence  between  meteorological  laws  and  river  floods, 
more  than  what  is  known  about  high  waters  produced  by  the  monsoons 
in  the  tropics,  and  the  locking  of  waters  by  frost  in  the  winter  in  north- 
ern latitudes. 

In  some  streams  dependent  on  water  from  the  melting  of  snow  in 
mountains,  there  is  a  perceptible  variation  in  the  stage  of  the  water 
having  the  period  of  a  day,  due  to  the  fact  that  more  snow  is  melted 
during  the  warm  than  the  cold  part  of  the  day.  The  time  of  highest 
stage  depends  on  the  distance  of  place  from  the  snow  field.  In  the  tem- 
perate zone  floods  occur  without  any  very  noticeably  great  rainfalls. 
Floods  in  a  river  are  apt  to  be  due  to  a  peculiar  sequence  of  rainfalls 
over  a  river  basin  rather  than  to  any  one  great  downpour.  Intermittent 
light  rains  may  cause  a  river  to  rise  slowly  and  steadily  until  it  is  nearly 
bank-full,  and  then  a  moderate  rain  but  little  greater  than  the  others 
will  carry  the  water  over  the  banks. 


RIVERS  AND  FLOODS.  209 

Flood  Combinations.  —  Floods  depend  largely  on  the  topographical 
features  of  a  country,  in  combination  with  sequence  of  rainfall  over  its 
various  parts.  A  succession  of  rainfalls  over  a  basin,  so  occurring  that 
the  flood  waters  from  a  number  of  tributaries  reach  some  part  of  the 
main  river  at  the  same  time,  will  give  a  high  stage  for  all  the  places 
along  its  lower  course.  A  difference  of  some  hours  in  the  times  of  rain- 
fall may  cause  the  floods  from  the  various  tributaries  to  pass  in  succes- 
sion through  the  main  river,  producing  a  medium  stage  of  water 
extending  over  a  long  time,  but  no  very  great  high  water.  For  any 
particular  river  basin  the  number  of  combinations  capable  of  producing 
a  flood  or  high  water  is  large,  but  the  probability  of  any  one  of  them 
occurring  is  small  in  the  case  of  many  rivers.  The  occurrence  of  many 
floods  may  therefore  be  considered  as  due  to  a  combination  of  favourable 
circumstances  or  as  purely  fortuitous.  Care  must  be  taken  in  drawing 
conclusions  from  flood  records  of  a  river  with  regard  to  changes  in  its 
regimen. 

Forests  and  Rivers.  —  The  clearing  of  forests  from  land,  the  exten- 
sion of  cultivation,  and  the  introduction  of  subsoil  drainage,  may  have 
some  effect  on  river  regimen,  tending  to  increase  or  diminish  the 
highest  water  stages  occurring  during  floods,  but  these  are  far  out- 
weighed by  other  accidental  circumstances.  The  forests  of  Maine  are 
growing  in  ground  covered  with  boulders  filled  in  with  leaf  mould  and 
moss.  When  this  permeable  soil  is  burned,  there  is  left  a  very  imper- 
meable soil,  which  may  change  river  regimen  without  any  change  of 
rainfall. 

Ploughed  ground  is  more  permeable  to  water  than  prairie  sod,  and 
must  act  to  restrain  and  store  up  the  water  which  will  then  in  great 
part  be  evaporated  or  will  run  to  the  rivers  more  slowly  than  would 
otherwise  be  the  case.  Forests  have  no  direct  restraining  effect  in 
diminishing  the  heights  of  floods.  This  has  been  shown  by  the  gaug- 
ing of  two  rivers  from  two  similar  areas  of  land  side  by  side  in  France, 
when  both  were  covered  with  forest  and  after  the  trees  had  been 
cleared  from  one  of  them.  The  maximum  stage  showed  no  depend- 
ence on  the  forest. 

Effect  of  Forests.  —  Forest  growth  over  the  drainage  basin  of  a 
river  diminishes  the  amount  of  silt  carried  to  the  streams,  especially 


2IO  METEOROLOGY. 

forest  on  hillsides.  Bushes  serve  equally  well,  and  preserve  the  soil 
from  being  washed  away  to  the  streams.  In  this  way,  plant  growths 
diminish  the  flood  heights  of  a  river,  by  diminishing  the  amount  of 
sediment  thrown  down  in  river  beds  in  places  where  the  current 
slackens.  The  gradual  rise  of  the  bottom  of  a  river  in  this  way,  by 
slow  sedimentation,  may  cause  overflows  of  the  banks  finally,  which 
otherwise  would  not  have  occurred.  The  sedimentation  of  rivers  in 
south-eastern  France  after  the  clearing  away  of  the  forests  from  the 
hillsides  was  so  great  that  many  disastrous  floods  occurred.  In  recent 
years,  since  reforestation,  the  rivers  have  cleared  the  channels  of  much 
of  the  deposit,  and  the  former  regimen  has  been  in  a  measure  restored. 

Mining  Silt.  —  The  bed  of  the  Sacramento  River  in  California  at 
Sacramento  City  is  20  feet  higher  than  it  was  in  1849,  owing  to  the 
debris  from  gold-mining  washed  down,  especially  that  from  hydraulic 
mining.  The  river  water  consequently  rises  20  feet  higher  than 
formerly.  Now  that  hydraulic  mining  has  stopped,  being  forbidden 
by  law,  the  bed  of  the  river  will  probably  be  scoured  out  some  and  its 
former  regimen  at  least  partially  restored.  Part  of  the  silting  of  the 
Sacramento  River  is  due  to  the  diminished  tidal  flow  through  the  river, 
owing  to  the  reclamation  of  swamp  land  by  building  of  levees. 

Cultivation  Silt.  —  The  Chattahoochee  River  in  Georgia  is  an  ex- 
ample of  a  stream  that  has  become  silted  by  the  extension  of  cultiva- 
tion, so  that  lands  are  now  overflowed  which  formerly  were  not. 

At  Augusta,  Ga.,  where  the  Savannah  River  is  notable  for  its  floods, 
there  has  been  no  great  increase  of  high  waters,  even  though  there  has 
been  a  great  extension  of  cultivation  in  the  drainage  basin  in  the  past 
century. 

In  the  Savannah  River  the  Yazoo  flood  of  1796,  and  the  Harrison 
flood  of  1840,  with  a  stage  of  37.5  feet,  were  nearly  as  high  as  the 
great  high  water  of  September  n,  1888,  which  was  38.7  feet. 

River  Records.  —  From  the  most  ancient  times  a  gauge,  called  a 
"  nilometer,"  has  been  maintained  on  the  Nile  for  indicating  the  stages 
of  water. 

River  gauges  are  now  maintained  at  many  places,  and  a  record  of 
the  daily  stages  of  water  kept  in  the  interest  of  navigation  and  for  the 
purpose  of  giving  warning  of  floods.  It  is  only  within  recent  times 


RIVERS  AND  FLOODS.  211 

that  much  attention  has  been  given  to  the  matter  of  river-gauge  records 
in  the  United  States.  Since  1871  a  number  of  river  gauges  have  been 
maintained  at  places  on  the  Mississippi  River  and  its  tributaries,  and 
records  of  the  water  stages  have  been  kept  up  under  the  auspices  of 
the  government.  These  records  are  published  by  the  Mississippi  and 
Missouri  River  Commissions  and  the  United  States  Weather  Bureau. 
Records  previous  to  1871  have  been  kept  at  only  a  few  places  by 
interested  individuals,  corporations,  or  city  governments. 

Weather-map  and  Flood.  —  No  sufficiently  satisfactory  estimate  of 
the  amount  of  a  coming  rainfall  over  a  drainage  area  can  be  made  from 
a  weather-map  to  be  of  any  use  in  making  a  flood  prediction.  It  is 
impossible  to  tell  just  where  the  rainfalls  attending  a  storm  will  occur, 
or  in  what  quantity  the  rain  will  fall.  In  case  the  rivers  of  a  region 
are  already  high,  as  shown  by  the  gauge-readings,  and  there  is  a  storm 
of  the  first  order  over  the  region,  important  conclusions  can  be  drawn. 
At  Augusta,  Ga.,  for  instance,  with  a  stage  of  20  feet  in  the  Savannah 
River  and  the  centre  of  a  West  India  hurricane  within  200  miles  of 
the  basin,  a  farther  rise  in  the  river  of  at  least  10  feet  may  be  antici- 
pated. For  a  rise  of  20  feet  at  Augusta  there  is  required  an  average 
of  2.2  inches  of  rainfall  in  the  three  days  preceding  at  Augusta, 
Atlanta,  Chattanooga,  Knoxville,  and  Charlotte.  The  least  rainfall 
producing  such  a  rise  has  been  1.6  inches.  For  a  24  to  28  foot  rise 
requires  a  three-day  rainfall  at  the  same  places  of  2.5  to  6.4  inches. 
The  34.5-foot  stage  on  July  31,  1887,  was  preceded  by  a  rainfall  of 
5.4  inches  at  Atlanta  and  Augusta.  The  rainfall  preceding  the  38.7- 
foot  stage  of  September  u,  1888,  was  3.7  inches. 

Predictions  without  Gauges.  —  Flood  predictions  are  rarely  made 
for  rivers  not  provided  with  gauges.  Unless  something  is  known 
about  the  customary  stages  of  water  in  a  river,  no  prediction  of  a  river 
stage  of  any  value  can  be  made,  on  the  basis  of  an  approaching  cyclone, 
nor  even  from  rainfall  already  on  the  ground  as  observed  by  rain  gauges. 

For  the  purpose  of  forming  a  rough  estimate  of  the  effect  of  rainfall 
on  the  river  stages  at  a  place,  the  rainfalls  are  entered  on  a  chart  show- 
ing lines  of  equal  time  of  travel  from  the  various  parts  of  a  drainage 
basin  to  the  place  by  the  river  routes.  A  map  of  this  kind  for  St. 
Louis,  Mo.,  is  shown  on  page  244,  the  heavy  lines  representing  the 


212  METEOROLOGY. 

points  from  which  water  reaches  St.  Louis  in  one,  two,  three,  etc.,  days. 
These  lines  are  for  medium  stages  of  water,  and  based  only  on  the  dis- 
tance from  St.  Louis  by  river,  regardless  of  the  varying  velocity  of  the 
water  in  different  parts  of  the  river  courses.  In  reality,  these  lines 
vary  a  good  deal  for  different  stages  of  water,  the  velocity  of  the  cur- 
rent being  greater  the  deeper  the  water.  Such  maps,  however,  though 
not  very  accurate,  serve  to  show  approximately  what  volumes  of  water 
may  be  expected  to  flow  by  a  place  when  the  rainfalls  over  different 
parts  of  the  drainage  area  are  known. 

Floods  in  rivers  in  the  United  States  from  a  single  rainfall,  causing 
a  river  to  rise  from  a  low  stage  to  the  flood  line  in  a  single  day,  are 
very  rare.  This  does,  however,  occur  occasionally,  as  in  the  case  of  the 
Black  Warrior  River  at  Tuscaloosa,  Ala.,  when  there  was  a  rise  of  65 
feet  in  one  night,  on  March  25,  1881. 

Rainfall  and  Rise.  —  From  a  lack  of  knowledge  of  the  nature  of  the 
ground  over  drainage  areas,  and  from  the  uncertainty  in  the  factors 
that  determine  what  part  of  a  rainfall  runs  into  the  rivers,  even  the 
observed  depth  of  rain  after  it  has  fallen,  is  not  of  any  very  great  ser- 
vice in  determining  a  river  rise,  except  in  cases  where  the  rainfall  is 
excessive.  When  the  ground  is  very  much  below  the  temperature  of 
freezing-point  in  winter  and  a  slow  rain  occurs,  the  ground  becomes 
covered  with  glare  ice,  and  a  subsequent  heavy  rainfall  goes  almost 
wholly  to  the  rivers,  and  may  produce  a  rise  seemingly  very  dispropor- 
tionate to  the  observed  depth  of  rainfall.  Moreover,  the  rainfall  differs 
so  much  in  neighbouring  places,  that  observations  at  only  a  few  points 
over  a  drainage  basin  are  not  of  great  value  in  river-stage  prediction. 
One  place  may  have  a  fall  of  three  inches,  and  at  a  place  20  miles 
away  there  may  be  only  a  fraction  of  an  inch,  or  none  whatever.  While 
great  river  rises  may  occur  with  less  than  one  inch  of  rainfall  in  twenty- 
four  hours,  in  general,  no  trustworthy  conclusions  with  regard  to  rises 
can  be  drawn  from  rainfalls  alone,  unless  in  excess  of  two  inches  in 
24  hours  for  at  least  two  stations  in  a  drainage  basin,  which  may 
enable  one  to  form  an  idea  of  the  area  covered  by  the  rainfall.  This 
applies  to  areas  of  not  more  than  5000  square  miles. 

Course  of  Rise.  —  In  a  small  drainage  basin  the  river  rise  after  a 
rainfall  is  very  rapid,  and  the  water  subsides  as  quickly  as  it  rises. 


RIVERS  AND  FLOODS.  213 

The  larger  the  drainage  basin,  the  slower  and  more  regular  the  rise 
and  fall. 

There  is  a  motion  of  a  flood  wave  down  stream.  The  crest  of  the 
wave  moves  with  a  velocity  dependent  on  the  slope  of  the  river  and  the 
mean  hydraulic  depth,  and  does  not  differ  a  great  deal  from  the  velocity 
of  the  water.  A  flood  wave  is  apparently  retarded  as  compared  with 
the  water  velocity,  due  to  the  water  having  to  fill  the  empty  river  chan- 
nel as  the  stage  increases.  A  flood  wave  from  the  Yellowstone  River 
takes  ten  days  to  reach  St.  Louis.  From  Cincinnati  to  Cairo  the  time 
of  a  flood  wave  is  6  days ;  from  Cairo  to  Vicksburg,  7  days ;  and  from 
Vicksburg  to  New  Orleans,  4  days. 

Flood  Crest.  —  After  rain  falls  and  flows  into  a  river,  it  has  the 
velocity  peculiar  to  the  slope  and  hydraulic  depth  of  the  stream  where 
it  enters.  The  first  additions  of  water  proceed  down  stream  at  a 
uniform  velocity.  As  more  water  is  added  from  the  same  area,  the 
hydraulic  depth  being  increased,  the  velocity  becomes  slightly  greater, 
and  in  the  course  of  time  the  water  catches  up  with  the  first.  As  water 
continues  to  be  added,  the  hydraulic  depth  and  the  velocity  continues 
to  increase,  and  the  water  overtakes  that  ahead  always  at  a  point  far- 
ther down  stream.  In  this  way  a  wave  crest  is  formed  and  reaches  its 
greatest  development  some  distance  down  stream  from  the  region  of 
rainfall.  In  the  case  of  widely  extended  rainfalls  over  large  drainage 
basins,  the  waters  from  the  various  tributaries  produce  the  flood-wave 
crest  in  the  main  river.  After  the  mean  hydraulic  depth  stops  increas- 
ing, there  is  a  gradual  spreading  out  of  a  flood  wave  and  a  lowering  of 
its  crest,  as  it  moves  down  stream,  when  there  are  no  further  additions 
from  tributaries. 

Velocity  of  Water.  —  The  velocity  of  the  water  in  mountain  streams 
or  torrents  is  very  different  from  that  in  streams  through  comparatively 
level  country.  In  torrents,  on  account  of  the  greater  steepness  of  the 
banks,  the  hydraulic  depth  increases  faster  with  the  stage  than  it  does 
in  rivers  with  very  sloping  banks.  While  the  ordinary  increase  in  the 
velocity  of  a  river  from  a  low  to  high  stage  is  not  usually  more  than 
double,  in  torrents  it  may  be  four  or  five  times  greater.  But  this  is 
very  variable  in  different  streams.  The  Missouri  River  from  low  to 
high  water  increases  in  velocity  (at  Omaha)  from  2.3  miles  to  5.3  miles 


214  METEOROLOGY. 

per  hour  for  the  average  of  the  cross-section.  The  average  velocity  of 
the  Ohio  River  (at  Paducah)  near  its  mouth  varies  from  one  mile  an 
hour  at  low  water  to  three  miles  at  high  water.  The  current  of  a  river 
is  swiftest  in  the  middle  of  a  stream  at  some  distance  below  the  surface, 
depending  slightly  on  the  velocity  and  direction  of  the  wind.  The  aver- 
age velocity  of  water  in  a  river  from  bottom  to  surface  throughout  a 
depth  is  equal  to  0.87  of  the  mid-depth  velocity.  Sometimes  the  average 
velocity  is  considered  to  be  at  six-tenths  of  the  depth. 

With  a  rising  stage  of  river  the  velocity  is  greater  than  for  the  same 
stage  when  the  river  is  falling.  In  gauging  a  river,  by  measuring  the 
velocity  with  a  current  meter,  a  coming  rise  is  often  indicated  on  the 
lower  Mississippi  48  hours  in  advance  by  an  increase  in  the  velocity 
of  the  water. 

Torrent  Velocity.  —  In  torrents  the  velocity  of  the  water  is  some- 
times as  great  as  50  feet  a  second,  or  34  miles  an  hour.  The 
tremendous  ravining  effect  of  such  a  current  can  be  understood  from 
the  fact  that  the  energy  of  impact  in  water  is  proportional  to  the  sixth 
power  of  the  velocity.  A  current  of  ten  feet  a  second  is  sufficient  to 
move  a  stone  weighing  four  pounds ;  when  the  velocity  is  doubled  it  is 
capable  of  moving  a  stone  sixty-four  times  as  heavy,  or  256  pounds.  In 
the  valley  of  the  Ardeche,  one  of  the  tributaries  of  the  Rhone  in  south- 
eastern France,  a  torrent  has  transported  a  rock  which  is  estimated  to 
weigh  450  tons. 

Rise  of  Torrents.  —  In  a  torrential  river  there  are  two  rises  in  a 
flood  ;  the  first  quickly  after  the  rainfall,  which  as  quickly  subsides  ; 
the  second  a  slow  and  steady  rise  from  the  water  oozing  out  of  the 
ground  from  numerous  springs. 

In  every  great  valley  there  is  a  point  where  floods  cease  increasing. 
This  is  where  the  flood  of  the  principal  stream  passes  without  meeting 
the  floods  of  affluents  already  past.  This  point  is  the  farther  from 
the  source  of  a  river,  the  more  mountainous  the  country,  because,  on 
account  of  the  greater  slopes  of  the  thalwegs,  the  floods  pass  over 
more  space  in  a  given  time  than  in  a  nearly  level  country. 

In  a  torrential  stream  a  great  flood  may  be  produced  by  a  single 
rainfall  over  only  a  part  of  its  basin,  and  not  necessarily  the  part  about 
its  source.  The  floods  in  small  torrents  do  not  usually  last  longer 
than  24  hours. 


RIVERS  AND  FLOODS.  215 

I 

The  tranquil  flow  which  follows  the  torrential  flood  increases  at 
every  confluence,  since  it  is  of  some  duration.  The  flood  then  goes 
on  increasing  at  each  confluence  until  it  reaches  one  where  all  the 
flood  has  passed,  from  which  point  the  height  begins  to  diminish. 

In  permeable  ground,  floods  of  the  affluents  concur  in  increasing 
the  flood  in  the  principal  stream.  The  duration  of  floods  in  such 
streams  does  not  increase  notably  as  they  progress.  In  torrents,  the 
duration  of  floods  being  very  short,  a  flood  most  always  passes  before 
another  comes  on.  Two  succeeding  floods  are  usually  independent  of 
each  other. 

River  Discharge.  —  A  knowledge  of  the  quantity  of  water  passing 
in  a  river  in  a  given  time,  or  the  river  discharge,  is  needed  in  esti- 
mating the  relative  importance  of  streams  in  producing  floods.  The 
discharge  is  commonly  given  in  cubic  feet  per  second  passing.  The 
discharge  depends  on  the  stage  of  water,  being  greater  the  higher 
the  stage.  The  discharge  in  cubic  feet  in  a  second  is  equal  to  the 
area  of  the  cross-section  of  a  stream  in  square  feet  multiplied  by  the 
velocity  of  current  in  feet  per  second. 

There  are  four  methods  of  measuring  the  velocity  of  a  river  current : 

First j  The  Weir  Method.  This  method  depends  on  measuring  the 
height  of  a  surface  of  water  running  over  a  weir ;  this  can  be  done 
with  very  great  accuracy  by  means  of  a  gauge  constructed  specially 
for  the  purpose.  The  method  is  only  applicable  to  small  streams  and 
is  the  most  accurate  of  any  method  of  stream  gauging. 

Second,  The  Meter  Method.  This  method  consists  in  ascertaining 
at  different  depths  in  the  different  parts  of  a  stream,  the  number  of 
revolutions  made  in  a  given  time  by  an  immersed  propeller  wheel,  an 
electrical  break-circuit  being  used  to  record  the  number  of  turns. 
The  velocity  corresponding  to  the  number  of  revolutions  is  ascer- 
tained by  dragging  the  meter  at  a  known  velocity  through  the  quiet 
water  of  a  pond.  This  is  the  most  satisfactory  method  of  measuring 
velocity  in  large  streams,  and  the  one  commonly  used.  By  means  of 
a  cable  stretched  across  a  stream  a  meter  can  be  carried  on  a  pulley 
to  any  desired  point  and  readily  lowered  to  any  part  of  the  river 
channel.  The  insulated  wires  connecting  the  wheel  with  a  register 
on  shore  record  the  number  of  turns ;  the  interval  of  time  for  which 


2l6  METEOROLOGY. 

the  turns  are  taken  is  usually  five  or  ten  minutes  for  each  point  where 
a  measurement  is  made.  In  the  case  of  navigable  streams,  the  stretch- 
ing of  a  cable  not  being  permissible,  the  meter  has  to  be  lowered  from 
a  boat  anchored  in  the  stream  for  the  purpose. 

Third,  The  Float  Method.  This  method  consists  in  observing  the 
velocity  of  water  by  means  of  an  object  floating  down  a  stream.  This 
is  done  by  observing  it  from  two  stations  on  shore,  the  distance 
between  them  being  known.  The  float  consists  of  a  keg  or  tin  vessel 
attached  to  a  rod  and  loaded  so  as  to  leave  a  little  of  the  stick  pro- 
jecting above  the  water  surface.  The  rod  takes  the  velocity  of  the 
water  at  the  place  of  the  vessel.  This  method  is  cumbersome  and 
at  the  present  time  rarely  used  where  extensive  gaugings  of  a  river 
are  to  be  made. 

Fourth,  The  Slope  Method.  This  method  consists  in  computing 
the  velocity  by  means  of  the  measured  slope  of  the  water  surface  and 
the  mean  hydraulic  depth  of  the  stream.  The  formula  depends  on 
observations  made  by  the  methods  described  above.  The  slope  of 
water  surface  is  determined  by  means  of  simultaneous  gauge  readings 
at  two  places  a  number  of  miles  apart,  the  difference  in  elevation  of 
the  marks  being  known  by  means  of  accurate  levellings. 

The  method  gives  only  a  general  approximate  result.  The  con- 
stant of  the  formula  is  very  uncertain,  depending  on  the  friction  of 
the  water  on  the  bed  of  the  river  and  its  various  irregularities  and 
also  on  the  magnitude  of  the  stream.  It  is  applicable  only  to  a 
straight  stretch  of  river  without  bends  or  curves.  It  is  uncertain 
what  length  of  river  should  be  used  in  deriving  the  slope  of  surface. 

In  the  following  tables  are  given  the  river  discharges  and  the 
amount  of  rainfall  in  cubic  miles  for  each  of  the  years  1881  and  1882, 
and  for  each  of  the  drainage  areas  of  the  upper  Mississippi  River,  the 
lower  Mississippi,  the  Ohio  River,  and  the  Missouri.  There  was  a  flood 
in  the  Missouri  River  in  1881,  and  in  the  Mississippi  in  1882.  The  dis- 
charges are  mainly  derived  from  measurements  with  a  propeller-wheel 
meter. 

A  discharge  of  one  cubic  mile  in  a  month  of  thirty  days  corresponds 
to  an  average  discharge  of  56,790  cubic  feet  per  second  during  the  time. 


RIVERS  AND   FLOODS. 


217 


RIVER  DISCHARGE  AND  RAINFALL  IN  CUBIC  MILES  OF  WATER. 

YEAR  1881. 


WHOLE 
MISSISSIPPI  RIVER. 

OHIO  RIVER. 

MISSOURI  RIVER. 

UPPER 
MISSISSIPPI  RIVER. 

MONTH. 

. 

Disch'ge. 

Rainfall. 

Disch'ge. 

Rainfall. 

Disch'ge. 

Rainfall. 

Disch'ge. 

Rainfall. 

January.     .     .     . 
February     .     .     . 

6.8 
H-5 

33-5 
58-3 

3-9 
7-5 

II.4 

1  6.0 

0.6 

1.2 

9.6 
10.8 

0.9 
I.I 

3-2 
8.7 

March    .... 

22.2 

36.0 

7-i 

12.0 

3-1 

8.7 

2-3 

5-5 

April      .... 

22.2 

40.4 

8.9 

12.5 

4-7 

10.9 

4.8 

3-9 

May  

22.2 

77.6 

4..Q 

0.6 

4.7 

26.5 

4.6 

o.e 

Tune       .... 

17.7 
II.  c 

72.2 

A  CO 

3-3 

2.2 

17.6 
8.6 

3-i 

2.7 

<-«»J 

26.9 

17.  1 

3-i 
3.1 

15-3 
10.6 

Tulv  . 

August  .... 

5-1 

34.2 

I.I 

4.6 

I.O 

13-0 

1.2 

9.1 

September  .     .     . 

4.2 

71.6 

I.I 

II.  2 

0.8 

21.4 

i-3 

19.2 

October.     .     .     . 

54 

69.7 

1.6 

144 

i-7 

I6.5 

4-3 

16.1 

November  .     .     . 
December  .     .     . 

9-9 
12.9 

45-i 
33-7 

3-2 
4.9 

1  8.  i 

1.6 
i.i 

6.7 
2.4 

6.2 
2.8 

7.2 
3-5 

Sums      .     .     . 

154.6 

614.2 

49-7 

151.1 

26.3 

I70-5 

35-7 

in.S 

RIVER  DISCHARGE  AND  RAINFALL  IN  CUBIC  MILES  OF  WATER. 

YEAR  1882. 


WHOLE 
MISSISSIPPI  RIVER. 

OHIO  RIVER. 

MISSOURI  RIVER. 

UPPER- 
MISSISSIPPI  RIVER. 

MONTH. 

Disch'ge. 

Rainfall. 

Disch'ge. 

Rainfall. 

Disch'ge. 

Rainfall. 

Disch'ge. 

Rainfall. 

January  .... 
February     .     .     . 

19.0 
22.1 

56.0 
58.9 

16.7 
17.6 

26.8 
21-7 

0.8 
1.2 

11 

1.8 
1.9 

2.6 
4.9 

March    .... 

29.6 

45-2 

16.2 

I6.7 

1.6 

7-4 

2.9 

7-3 

28.4 

54.1 

6.9 

IO.O 

i-7 

20.5 

3-9 

8.5 

May       .... 

24.8 

98.4 

8.5 

21.0 

2.0 

29-3 

5-° 

13-4 

22  Q 

74.  O 

80 

TCI 

2  8 

27  6 

July  .          .     , 

21  7 

67.8 

4.  2 

»3«» 

1^8 

3  A. 

•*/•" 
IQ  7 

Is 

17.9 
o  6 

August  .... 

I2.I 

52.1 

Is 

20.6 

J'4 

i-3 

*:*•/ 

7.0 

4.0 
1.8 

y.u 

7.8 

September  .     .     . 

6.1 

38.7 

2-5 

"•3 

0.6 

9.9 

1.2 

3-5 

October.     .     .     . 

4.9 

48.1 

1.4 

6-4 

0.6 

14.6 

I.O 

IO.I 

November  .     .     . 
December  .     .     . 

£ 

37-2 
25-5 

1-3 

2.1 

8.9 
7-9 

0.7 
0.4 

5.8 
7.0 

1-3 

I.O 

5.4 
4-3 

Sums      ... 

202.9 

656.0 

88.2 

180.2 

17.1 

'59-9 

32.0 

95-3 

CHAPTER   X. 

RIVER-STAGE    PREDICTIONS. 

Freshet  Waves  in  Rivers.  —  A  high-water  wave  in  a  river  being  pro- 
gressive, some  idea  can  be  formed  in  advance,  as  to  the  stages  of  water 
that  will  occur  along  the  lower  course  of  a  river,  when  the  stages 
at  points  along  the  upper  course  are  known.  The  judgment  is  based 
on  what  has  been  observed  to  have  occurred  in  preceding  cases  of  high 
waters.  Hence  the  value  of  a  record  of  water  stages,  to  determine  the 
relation  between  the  wave  crests  along  a  river  course.  The  relation  of 
wave  crests  is  not  identically  the  same  in  all  cases  of  high  water.  It 
depends  on  the  distribution  of  rainfall  over  the  drainage  basin  of  a 
river.  The  average  of  a  great  many  cases,  however,  gives  a  result 
which,  though  sometimes  in  error,  yet  in  many  or  most  cases  is  very 
nearly  right.  The  result  is  uncertain  by  the  amount  of  water  entering 
the  river  between  gauges,  which  goes  to  swell  that  passing  the  lower 
gauge  without  affecting  the  stage  at  the  upper  one. 

When  the  relation  of  the  high-water  stages  at  two  places  is  known 
the  observed  stage  at  the  upper  one  can  be  used  to  predict  the  stage 
that  will  occur  at  the  lower  one.  The  prediction  will  be  the  more  accu- 
rate the  more  of  the  drainage  basin  there  is  above  the  upper  gauge,  and 
the  nearer  the  two  gauges  are  together. 

If,  however,  the  upper  gauge  is  close  to  the  lower  one,  a  predic- 
tion loses  in  value,  on  account  of  the  slight  interval  of  time  between 
the  prediction  of  a  stage  and  its  occurrence. 

In  a  river  draining  a  very  great  area  and  without  any  affluents, 
stages  can  be  predicted  several  days  ahead  accurate  to  within  a  few 
inches.  When  a  rise  at  a  place  is  the  result  of  rises  in  several  tribu- 
taries, a  stage  cannot  be  predicted  with  as  great  an  accuracy.  It 
may  be  in  error  in  some  cases  two  or  three  feet  or  even  more,  depend- 
ing on  the  extent  of  drainage  basin  and  the  number  of  tributaries. 

218 


RIVER-STAGE  PREDICTIONS.  2 19 

Methods  of  Predicting  Stages.  —  To  derive  rules  or  methods  for 
predicting,  river  stages  requires  a  long-continued  record  of  the  stages 
of  water  at  two  or  more  points  on  a  river.  In  order  to  derive  a  rule 
for  rivers  with  many  tributaries,  a  record  extending  over  several  years 
is  necessary,  with  rises  occurring  sometimes  in  one  tributary  and 
sometimes  in  another,  in  order  that  the  effects  of  the  various  tribu- 
taries in  producing  a  rise  in  the  main  river  may  be  disentangled. 

A  theoretical  determination  of  a  river  rise  as  a  problem  in  dynamics 
or  hydraulics  is  entirely  out  of  the  question.  The  complexity  of  such  a 
problem  is  very  great,  involving  the  varying  depth,  slope,  and  cross- 
section  of  a  river  in  different  parts  of  its  course  and  the  tortuousness  of 
its  channel.  Moreover,  a  precise  result  is  not  to  be  expected,  because 
of  the  water  entering  the  river  between  gauges. 

Where  there  are  two  gauges  on  a  river,  without  any  large  tributary 
entering  the  river  between  them,  the  method  of  rinding  the  relation 
between  stages  is,  to  take  the  mean  of  all  the  high-water  crests  about 
a  certain  stage  for  the  upper  gauge,  and  compare  it  with  the  mean  of 
the  corresponding  highest  stages  that  are  found  to  follow  at  the  lower 
gauge.  From  these  means  a  table  can  be  derived,  giving  the  corre- 
sponding stages  at  the  two  places,  proceeding  by  differences  of  one  foot 
from  the  highest  to  the  lowest  for  the  upper  gauge.  These  correspond- 
ing stages  will  differ  in  time  one  day,  or  several  days,  as  the  case  may 
be,  depending  on  the  distance  of  the  gauges  apart. 

The  method  of  finding  the  relation  between  crests  on  rivers  with 
tributaries  coming  in  between  gauges  varies  with  the  nature  of  the 
tributaries  and  on  the  records  available  for  deriving  a  rule.  Various 
suppositions  are  made  as  to  the  relation  of  the  rises,  until  something  is 
found  that  satisfies  tolerably  well  all  the  observed  high  waters.  An 
examination  of  the  rainfall  records  is  made,  and  the  rises  excluded, 
which  are  found  to  be  dependent  mainly  on  rainfall  above  the  upper 
station  or  in  the  drainage  area  between  the  two  gauge  stations.  When 
the  record  of  gauge  readings  for  a  place  extends  over  a  number  of  years, 
and  the  rises  are  numerous,  some  idea  can  be  formed  of  the  accuracy  of 
a  predicted  stage.  The  method  of  comparative  crests  used  for  rivers 
without  great  tributaries  is  also  available  for  rivers  with  them  to  a 
limited  extent  when  no  better  method  is  available,  the  number  of  rises 
to  work  with  being  small. 


220  METEOROLOGY. 

Where  a  gauge  record  is  kept  usually  at  least  one  reading  is  taken 
daily  at  the  same  hour  every  day. 

Gauge  Readings  at  a  Single  Place.  —  Gauge  readings  at  a  single  point 
on  a  river,  when  made  daily  or  hourly  during  a  rise,  are  of  value  in  many 
cases  in  forming  an  idea  of  what  a  stage  will  be  in  the  near  future. 
Usually  something  is  known  as  to  the  average  duration  of  rises  at  a 
place.  At  Cincinnati,  for  instance,  important  rises  continue  at  least  six 
days.  The  variation  of  rise  where  the  gauge  is  of  such  a  character  that 
the  stages  can  be  accurately  observed,  is  of  great  service  in  estimating 
a  coming  stage  of  water  a  short  time  ahead.  By  means  of  observations 
at  intervals  of  two  or  three  hours  it  can  be  ascertained  whether  the  rate 
at  which  the  water  surface  is  rising  is  increasing  or  diminishing.  When 
the  rate  of  rise  is  diminishing,  the  high-water  crest  cannot  be  far  dis- 
tant. If,  for  instance,  the  gauge  readings  at  a  place  at  intervals  of  four 
hours  show  rises  of  four  feet,  three  feet,  and  two  feet :  then  it  may  be 
inferred  that  the  rise  in  the  next  four  hours  will  not  be  more  than  one 
foot,  when  the  crest  will  be  reached  and  the  stage  will  begin  to  diminish. 

Gauge  readings  at  a  single  place  can  only  be  used  to  advantage  in 
predicting  stages  when  the  area  of  drainage  basin  above  the  place  is 
very  considerable,  at  least  30,0x30  square  miles.  Even  with  such  a  large 
area  there  is  often  much  irregularity  in  the  rises,  so  that  any  rules  that 
may  be  derived,  based  on  observed  rates  of  rise  alone,  are  of  limited 
value. 

The  average  daily  rises  at  Cincinnati  are  as  follows,  for  six  days  pre- 
ceding the  high-water  crests  as  derived  from  59  cases  since  the  year 
1858,  where  the  rise  was  at  least  15  feet,  and  the  highest  water  as  great 
as  40  feet :  — 

Days  before  crest,         6  to  5       5  to  4       4  to  3       3  to  2       2  to  i       i  to  crest 
Rise  in  feet,  2.2  3.2  3.8  4.0  2.8  1.3 

For  St.  Louis  the  rises  for  like  intervals  before  a  crest  are  as 
follows  :  — 

Rise  in  feet,  0.3  0.4  0.6  0.9  i.o  0.5 

The  rate  of  rise  increases,  on  the  average,  from  the  sixth  to  the  third 
day  before  a  crest,  and  diminishes  from  the  third  day  to  the  day  of  the 
crest.  At  St.  Louis  the  greatest  rate  of  rise  is  only  one-fourth  as  great 


RIVER-STAGE  PREDICTIONS.  221 

as  at  Cincinnati.  The  rate  of  rise  is  more  apt  to  be  irregular  in  the 
case  of  great  rises  than  small  ones. 

Floods  at  Paris,  France.  —  The  rises  in  the  Seine  River  at  Paris  are 
predicted  three  days  in  advance,  from  gauge  readings  made  at  seven 
places  on  streams  in  the  upper  part  of  the  drainage  area  of  the  Seine  : 
at  Clancy  on  the  Yonne,  at  Avallon  on  the  Courson,  at  Aisy  on  the 
Armancon,  at  Chaumont  on  the  Marne,  at  Vraincourt  on  the  Aire,  and 
at  Sainte  Menehould  on  the  Aisne.  The  last  two  places  are  not  in  the 
part  of  the  Seine  basin  above  Paris,  but  are  so  close  to  the  Marne,  and 
the  rain  conditions  over  the  drainage  areas  are  so  similar,  that  the  rises 
are  indicative  of  rises  in  the  upper  Seine.  These  stations  are  represen- 
tative of  the  impermeable  area  of  the  Seine  basin  which  is  the  most 
important  part  of  the  drainage  area  in  flood  production.  The  stations 
probably  also  represent  country  of  high  average  slope.  Either  cause, 
impermeability  or  great  slope,  will  explain  Belgrand's  observations  on 
the  wetted  perimeters  of  the  stone-arch  bridges  of  the  Seine  valley. 

When  the  torrents  at  these  seven  places  show  a  rise,  and  at  the  same 
time  the  river  at  Paris  is  in  a  rising  stage,  the  average  of  the  seven  rises 
divided  by  the  number  1.99  gives  the  rise  that  may  be  expected  in  the 
next  three  days  at  Paris.  The  error  in  the  stage  computed  in  this  way 
is  never  greater  than  two  feet.  When  the  river  at  Paris  is  in  a  falling 
stage,  the  mean  rise  in  the  torrents  divided  by  1.46  gives  the  three-day 
rise  at  Paris.  The  maximum  error  of  a  computed  stage  in  this  latter 
case  is  somewhat  greater  than  in  the  first. 

The  average  duration  of  a  rise  at  Paris  is  3.4  times  that  of  a  rise  at 
the  upper  points.  The  number  of  days  a  flood  continues  is  usually 
three  or  four.  In  five  cases  out  of  eighty-one,  floods  have  lasted  five 
days.  The  duration  of  a  rise  at  Paris  depends  on  the  number  of  afflu- 
ents in  flood.  When  the  rise  lasts  six  to  eight  days  it  is  the  result  of 
floods  in  two  affluents  ;  when  it  lasts  nine  to  twelve  days  there  are 
floods  in  three  affluents ;  for  thirteen  to  fifteen  days  there  are  floods  in 
four  affluents.  The  number  of  affluents  participating  in  a  flood  in  dif- 
ferent cases  varies  from  one  to  four. 

There  is  a  record  of  high  waters  that  have  occurred  at  Paris  since 
the  year  1615.  On  July  nth  of  that  year  the  water  reached  a  height 
of  29.3  feet  above  low  water,  a  stage  that  has  never  since  been  equalled. 


222  METEOROLOGY. 

The  gardens  of  the  Tuileries  were  flooded.  In  January,  1658,  a  stage 
within  half  a  foot  of  the  highest  was  reached.  Stages  of  water  as  high 
as  23  feet  occur,  on  the  average,  about  once  in  25  years. 

The  combinations  of  flood  waves  from  the  tributaries  of  the  Seine 
that  produce  the  greatest  floods  at  Paris  are  due  to  a  peculiar  distribu- 
tion of  rainfall  over  the  various  drainage  areas  that  may  be  considered 
as  purely  accidental.  Great  floods  may  occur  at  intervals  of  a  few  years 
and  not  in  a  thousand  years. 

This  method  of  predicting  high  stages  of  water  at  Paris  was  devised 
by  Belgrand,  and  has  been  in  use  since  1854. 

Regular  predictions  of  water  stages  are  now  made  in  France  for  many 
places  along  the  Saone,  the  Loire,  and  the  Garonne  rivers.  For  some 
points  where  there  are  no  tributaries  coming  in  between  gauges,  the 
predicted  stages  are  never  in  error  more  than  four  inches.  The  rules 
for  making  predictions  are  in  some  cases  based  on  observed  rises,  and 
in  others  on  comparison  of  corresponding  stages  at  two  or  more  places. 

Attempts  that  have  been  made  in  France  to  use  discharge  measure- 
ments in  predicting  river  stages  have  not  as  yet  proved  successful.  By 
discharge  measurements  is  meant  the  rate  of  flow  in  cubic  feet  per 
second,  as  dependent  on  the  varying  stage  of  a  river.  The  best  method 
of  predicting  stages  has  been  found  to  be  by  comparison  of  dependent 
rises  at  different  places. 

Very  little  has  been  done  as  yet  in  any  country,  except  France,  in 
the  way  of  developing  methods  of  predicting  river  rises. 

River-Stage  Predictions  at  Pittsburg. —  For  the  drainage  area  above 
Pittsburg,  the  observations  of  rainfall  cannot  be  used  to  any  great 
advantage  to  foretell  with  any  accuracy  to  what  stage  the  river  will 
rise.  The  rainfalls  over  the  drainage  area,  which  is  17,000  square  miles, 
are  not  uniform  in  depth,  nor  do  they  occur  simultaneously  over  the 
various  parts  of  the  area.  Inasmuch  as  the  variations  in  depth  of  rain- 
fall over  the  different  parts  are  so  great,  and  the  time  of  travel  of  the 
water  to  Pittsburg  so  short,  a  discussion  of  the  relation  of  rainfall  to 
river  stage  would  only  be  of  small  value.  This  is  especially  so,  since 
the  observations  of  rainfall  preceding  the  occurrence  of  great  rises  are 
not  very  plentiful,  and  the  volume  of  river  discharge  for  different  stages 
of  the  river  is  not  known. 


RI VER-S TA  GE  PREDICTIONS. 


223 


The  most  that  can  be  inferred  from  rainfall  observations  is  about  as 
follows :  When  the  rainfall  on  three  successive  days  for  the  average  of 
20  stations  in  the  drainage  basin,  as  shown  on  map  accompanying,  is  i.o 
inch,  the  stage  reached  will  be  22  feet;  for  a  three-day  rainfall  of  2.5 
inches  on  the  average  at  20  stations,  the  stage  attained  will  be  31  feet. 

The  most  satisfactory  method  of  estimating  a  high  stage  at  Pittsburg 
is  by  means  of  the  rises  at  stations  on  the  rivers  above  it.  For  this 
purpose  the  rises  at  Oil  City,  Brookville,  Confluence,  Rowlesburg,  Wes- 

RIVER  GAUGE  AND  RAINFALL  STATIONS  ABOVE  PITTSBURG,  PA. 


79* 


78* 


79° 


I 

2 

3 
4 

5 
6 

7 
8 

9 
10 
ii 

12 

13 

14 
15 

16 

17 
18 

19 

20 
21 
22 

23 
24 

25 


STATIONS. 


Warren,  Pa. 

Oil  City,  « 

Clarion,  " 
Parker's  Landing,   " 

Brookville,  " 

Mahoning,  " 

Johnstown,  " 

Saltsburg,  " 

Freeport,  " 

Confluence,  " 

West  Newton,  " 
Weston, 
Philippi, 
Fairmont, 
Morgantown, 
Rowlesburg, 
Greensboro, 

Lock  No.  4,  " 

Pittsburg,  " 
Davis  Island  Dam,  " 

Ridgway,  " 

Buchannon,  W.  Va. 

New  Castle,  Pa. 

Scoyestown,  il 

Du  Bois,  " 


W.  Va. 
u 


Pa. 


224 


METEOROLOGY. 


ton,  and  Johnstown,  are  used  because  of  the  greater  lengths  of  record 
available  at  those  places  for  deriving  the  relation  of  stages.  The  drain- 
age areas  above  these  places  are  respectively  4526,  400,  782,  886,  140, 
and  711  square  miles.  Rises  at  these  places  have  different  power  to 
produce  rises  at  Pittsburg.  The  power  to  produce  a  rise  at  Pittsburg 
will  be  assumed  to  be  in  proportion  to  the  square  root  of  the  areas 
above  them.  Taking  the  unit  of  area  as  1000  square  miles,  the  relative 
weights  of  the  rises  at  the  different  places  in  producing  a  rise  at  Pitts- 
burg will  be  for  Oil  City,  2.  i  ;  Brookville,  0.6 ;  Confluence,  0.9 ;  Rowles- 
burg,  0.9;  Weston,  o.i ;  Johnstown,  0.8. 

Rises  at  any  one  or  all  of  these  places  have  different  power  to  pro- 
duce rises  at  Pittsburg  depending  on  the  stage  at  Pittsburg ;  the  higher 
the  stage  at  Pittsburg  the  less  will  be  the  rise,  the  rises  at  upper  sta- 
tions being  the  same  in  both  cases.  It  is  assumed  that  the  rise  mul- 
tiplied by  the  mean  stage  during  the  rise  is  comparable  throughout 
different  stages  for  Pittsburg. 

In  accordance  with  these  assumptions,  the  table  below  is  derived,  in 
which  is  given  the  stage  that  will  be  reached  at  Pittsburg  when  the 
rises  at  places  above  it  are  known.  The  horizontal  argument  6,  8,  10, 
12,  etc.,  is  the  stage  at  Pittsburg  at  the  beginning  of  a  rise;  the 
vertical  argument  15,  20,  25,  etc.,  is  the  sum  of  the  weighted  rises  at 
six  stations,  that  is,  the  rise  at  Oil  City  multiplied  by  2. 1,  Brookville 
by  0.6,  Confluence  by  0.9,  Rowlesburg  by  0.9,  Weston  by  0.1,  and 
Johnstown  by  0.8,  and  all  the  products  added  together.  The  figures  in 
the  body  of  table  are  the  highest  stages  to  be  expected  at  Pittsburg. 

CREST  STAGES  AT  PITTSBURG. 


WEIGHTED  SUM 
OF  RISES  AT 
Six  STATIONS. 

PITTSBURG  STAGE  AT  BEGINNING  or  RISE. 

6 

8 

10 

12 

14 

16 

18 

20 

22 

15 

18 

19 

20 

21 

22 

23 

25 

27 

28 

20 

21 

22 

22 

23 

24 

26 

27 

29 

30 

25 

23 

24 

24 

25 

26 

28 

29 

30 

31 

30 

25 

26 

26 

27 

28 

29 

30 

32 

33 

35 

27 

28 

28 

29 

30 

31 

32 

40 

29 

30 

3° 

3' 

31 

45 

30 

31 

32 

RIVER-STAGE  PREDICTIONS.  22$ 

No  predictions  should  ever  be  made  of  a  stage  higher  than  33  feet. 
Examples  of  the  use  of  the  table,  February  6,  1893  :  The  stage  at 
Pittsburg  was  10.0  and  rising.  The  stages,  February  6  and  7,  were, 
at  Oil  City,  6.8,  u.o;  Brookville,  3.3,  7.1;  Confluence,  4.8,  10.0; 
Rowlesburg,  4.5,  9.6 ;  Weston,  5.5,  do ;  and  at  Johnstown,  1.5,  2.5.  The 
weighted  sum  of  rises  is  :  — 

4.2x2.1 

3.8x0.6 

5.2x0.9 

5.1  xo-9 

0.5  xo.i 

1.0x0.8  =  21.2 

At  the  intersection  of  10  in  the  horizontal  heading,  and  at  one-fourth 
of  the  way  from  20  to  25  in  the  vertical  column,  the  stage  of  22+0.5 
is  found,  or  23  may  be  taken  as  the  highest  stage  to  be  expected.  The 
stage  reached  was  23.1. 

At  Pittsburg,  February  16,  1891,  the  stage  was  9.3  feet.  February 
1 6  and  17  the  stages  were,  at  Oil  City,  3.5,  16.2;  at  Brookville,  2.5, 
10.0;  at  Confluence,  6.9,  12.3 ;  at  Rowlesburg,  5.0,  5.7;  at  Weston,  3.5, 
3.0;  and  at  Johnstown,  7.3,  17.1. 

The  weighted  sum  of  rises  is  44.5.  At  the  intersection  of  the  vertical 
column  45,  and  the  horizontal  heading  9,  half-way  between  8  and  10, 
the  tabular  number  31+0.5  is  found,  or  the  highest  stage  to  be  expected 
may  be  considered  as  32  feet.  The  stage  was  31.3  feet. 

The  agreement  in  all  cases  will  not  be  as  good  as  in  the  examples 
given.  The  stages  estimated  in  this  way  may  at  times  be  in  error  by  as 
much  as  4  or  5  feet. 

Ohio  River,  Wheeling,  W.Va.  —  A  high-water  crest  at  Pittsburg  is 
followed  one  day  later  by  a  crest  at  Wheeling.  The  flood  line  is  at 
36  feet.  The  highest  water,  that  of  February  7,  1884,  was  52  feet. 

Ohio  River,  Parkersburg,  W.Va.,  and  Marietta,  Ohio.  —  A  high- 
water  crest  at  Pittsburg  is  followed  two  days  later  by  a  crest  at 
Parkersburg,  243.5  miles  below.  The  stages  at  Marietta,  12.5  miles 
above  Parkersburg,  and  at  Parkersburg  are  nearly  identical. 

The  flood  line  at  Marietta  is  at  25  feet.  The  highest  water,  that  of 
February  9,  1884,  was  55  feet. 


226 


METEOROLOGY. 


HIGH-  WATER  CRESTS. 

WHEELING. 

PARKERSBURG. 

IO 

I6.7 

15 

21-3 

21.6  ±  1.5 

20 

28.4  ±  1.4 

28.7  ±  3.0 

25 

25.6  ±  1.4 

37-5  ±  2-2 

3° 

44-5 

45-7 

32 

49.0 

48.7 

Ohio  River,  Cincinnati,  Ohio.  —  For  predicting  rises  of  the  river  at 
Cincinnati  there  are  available  the  stages  of  water  at  Parkersburg,  286 
miles  above ;  at  Charleston,  W.Va.,  on  the  Great  Kanawha  River,  235 
miles  above;  and  at  Louisa,  Ky.,  on  the  Big  Sandy  River,  178  miles 
above.  The  stage  of  water  at  Circleville,  Ohio,  on  the  Scioto  River  is 
also  observed,  but  it  bears  no  important  part  in  the  high  waters  that 
occur  at  Cincinnati.  The  relation  of  the  gauges  is  shown  on  the  map. 

There  is  a  record  of  the  daily  river  stages  at  Cincinnati,  beginning  in 
1858;  at  Marietta,  beginning  1873;  at  Charleston,  beginning  1873;  and 
at  Louisa,  beginning  1883. 

The  drainage  area  of  the  Ohio  River  above  Cincinnati  is  78,000 
square  miles.  Of  this  there  is  above  Parkersburg  36,000  square  miles ; 
above  Charleston,  12,000;  and  above  Louisa,  4000.  The  drainage 
areas  in  the  mountainous  region  of  Pennsylvania,  West  Virginia,  and 
Kentucky  are  the  important  ones  in  the  production  of  flood  stages  at 
Cincinnati.  Over  these  areas  there  is  great  average  slope  of  the 
ground,  and  it  is  mainly  impermeable  on  Belgrand's  classification. 

The  dominating  cause  of  a  rise  at  Cincinnati  is  a  preceding  rise  at 
Parkersburg.  In  103  cases  since  1873  the  wave  crest  at  Cincinnati 
occurred  with  respect  to  the  crest  at  Parkersburg  as  follows :  In  i  case 
6  days  after,  in  6  cases  4  days,  in  39  cases  3  days,  in  25  cases  2  days,  in 
1 8  cases  i  day,  and  in  10  cases  on  the  same  day.  The  wave  crest 
occurred  earlier  than  at  Parkersburg,  in  2  cases  I  day,  in  i  case  2  days, 
and  in  I  case  3  days.  The  difference  in  wave  time  between  the  two 
places  on  different  occasions  is  due  to  the  compounding  of  wave  crests 
from  the  tributaries  in  different  ways. 


RIVER-STAGE  PREDICTIONS. 


227 


The  best  that  can  be  done  will  be,  to  use  the  rises  and  stages  after 
a  crest  has  occurred  at  Parkersburg  in  predicting  the  rise  and  the  stage 
three  days  later  at  Cincinnati. 

Nothing  is  known  about  the  cross-sections  of  the  rivers  at  the  vari- 
ous places,  or  about  the  velocity  or  discharge  at  different  stages.  The 


same  stage  on  different  rivers  corresponds  to  very  different  quantities 
of  water  passing.  For  like  stages  at  two  places,  the  quantity  of  water 
passing  a  place  probably  has  some  relation  to  the  area  which  the  water 
drains. 

A  rise  of  ten  feet  at  Parkersburg  when  the  stage  is  high  will  cause 
a  greater  rise  at  Cincinnati  than  ten  feet  when  the  stage  is  low,  the 


228  METEOROLOGY. 

stage  at  Cincinnati  being  the  same  in  both  cases.  A  rise  at  Parkers- 
burg  will  have  a  greater  effect  in  producing  a  rise  at  Cincinnati,  the 
less  the  Cincinnati  stage,  and  a  less  effect  the  greater  the  stage  at 
Cincinnati. 

For  the  purpose  of  deriving  the  relation  between  the  rises  at  the 
upper  gauges  and  at  Cincinnati,  the  following  proceeding  was  adopted  : 
The  rise  at  Cincinnati  in  the  three  days  preceding  a  crest  multiplied  by 
the  stage  three  days  before  the  crest,  and  also  by  an  unknown  factor, 
is  placed  equal  to  the  sum  of  the  products  of  the  rises  at  Parkersburg, 
Charleston,  and  Louisa  from  the  sixth  to  the  third  day  before  the 
Cincinnati  crest,  multiplied  by  the  mean  stages  at  the  places  during 
the  time.  Separating  these  products,  as  derived  from  the  various  high 
waters,  into  groups  arranged  according  to  the  magnitude  of  the  stage 
of  water  at  Cincinnati,  the  unknown  factor  is  derived  for  a  number  of 
stages.  The  factor  is  considered  as  applying  to  the  mean  of  all  the 
Cincinnati  stages  from  which  it  is  derived.  The  factors  found  for 
the  various  stages  are  subjected  to  an  adjustment  for  the  purpose  of 
smoothing  out  irregularities. 

The  rule  thus  derived  for  finding  the  highest  stage  of  water  for  a 
flood  wave  at  Cincinnati,  or  a  three-day  rise  near  the  time  of  a  flood 
wave,  is  as  follows :  When  the  river  is  in  a  rising  stage  at  Cincinnati, 
and  has  been  rising  for  at  least  three  days,  and  a  crest  has  occurred  at 
Parkersburg,  then  the  rise  at  Cincinnati  in  the  next  three  days  will  be 
equal  to  the  previous  three-day  rise  at  Parkersburg  multiplied  by  the 
mean  stage  at  Parkersburg  on  the  day  and  the  third  day  preceding, 
plus  the  rise  at  Charleston  in  the  preceding  three  days  multiplied  by 
the  mean  stage,  plus  the  three-day  rise  at  Louisa  multiplied  by  the 
mean  stage,  the  sum  divided  by  the  stage  at  Cincinnati  and  multiplied 
by  the  factor  given  below,  depending  on  the  Cincinnati  stage. 


RIVER-STAGE  PREDICTIONS. 


229 


CINCINNATI. 

CINCINNATI. 

RIVER  STAGE. 

FACTOR. 

RIVER  STAGE. 

FACTOR. 

Feet. 

Feet. 

40 

0-43 

26 

0.97 

41 

0.42 

27 

0.87 

42 

0.4I 

28 

0.83 

43 

29 

0.79 

44 

0.38 

3° 

0.75 

45 

0-37 

3i 

0.71 

46 

0.36 

32 

0.67 

0-35 

33 

0.63 

48 

0-34 

34 

0-59 

49 

0-34 

11 

0.51 

50 
Si 

0.34 
0-34 

11 

0.48 
0.46 

52 

0-34 

39 

0.44 

In  case  there  is  a  fall  at  any  of  the  three  places,  it  enters  the  sum 
with  a  minus  sign. 

The  rule  can  still  be  used  for  deriving  the  approximate  rise  at  Cin- 
cinnati, even  when  the  crest  at  Parkersburg  is  three  days  past,  provided 
the  stage  at  the  other  places  is  very  high,  as  much  as  46  feet  and  rising. 
The  following  shows  some  of  the  important  rises  at  Cincinnati  com- 
puted according  to  this  rule  :  — 


DATE. 

CINCINNATI 
RIVER  STAGE. 

RISE  IN 
THREE  DAYS. 

COMPUTED 
RISE. 

ERROR. 

Feet. 

Feet. 

Feet. 

February  n,  1884      .     .     . 

66.8 

4-3 

3-0 

-i-3 

March  14,  1884     .... 

48.3 

i-3 

0.7 

-0.6 

January  17,  1885  .... 

4I.I 

4-9 

6.7 

+  1.8 

April  6,  1886    

C4..2 

1.6 

•j.-j 

4-  1.7 

January  18,  1890  .... 

40.3 

3-5 

3-5 

0.0 

February  26,  1890     .     .     . 

49.4 

74 

7-7 

+  0.3 

March  23,  1890     .... 

52.0 

7-i 

7-4 

+  0.3 

230  METEOROLOGY. 

It  is  not  to  be  expected  that  the  stage  of  water  given  by  this  method 
will  always  be  exactly  correct.  The  area  from  which  water  drains  pass- 
ing Cincinnati,  but  which  does  not  pass  Parkersburg,  Charleston,  or 
Louisa,  is  26,000  square  miles.  In  case  most  of  a  rainfall  is  above 
these  places,  the  computed  stage  will  be  too  high  ;  in  case  most  of  it 
is  below  the  places  the  computed  stage  will  be  too  low.  As  a  general 
thing,  rainfall  is  distributed  nearly  uniformly,  and  the  method  repre- 
sents the  average  of  rises  somewhat  nearly.  There  is  also  some  uncer- 
tainty in  a  predicted  stage  from  the  rainfall  that  may  occur,  in  the 
three  days  after  the  prediction,  in  the  immediate  vicinity  of  Cincinnati. 

The  flood  line  at  Cincinnati  is  at  45  feet.  The  highest  water,  that 
of  February  14,  1884,  was  71.1  feet.  This  high  stage  was  due  to  heavy 
rainfall  over  frozen,  ice-glazed  ground,  most  of  the  water  passing  imme- 
diately to  the  streams. 

The  following  is  the  method  of  using  the  rule  to  compute  a  river 
stage.  March  23,  1890,  the  river  at  Cincinnati  was  at  a  stage  of  52.0 
feet,  and  had  been  rising  for  more  than  three  days.  At  Parkersburg, 
where  the  river  was  at  a  crest  on  March  23d,  there  was  a  rise  in  three 
days  from  the  stage  of  19.0  to  31.0  feet.  At  Charleston  there  had 
been  a  rise  from  16.3  to  30.9  feet.  At  Louisa  there  had  been  a  rise 
from  24.7  to  40.0  feet.  Hence  the  rise  to  be  expected  at  Cincinnati 
according  to  the  rule  is  :  — 

0  34  [25  x  12.0  +  24  x  14.6  +  32  x  15.3]  =  7  4 

The  number  0.34  is  taken  from  the  table  of  factors  corresponding  to  52 
feet.  The  observed  rise  was  7.  i  feet. 

Ohio  River,  Louisville,  Ky.;  Evansville,  Ind.;  Mount  Vernon,  Ind.  — 
The  time  of  a  flood  wave  from  Cincinnati  to  Louisville  is  one  day ;  the 
distance  is  132  miles.  The  Kentucky  River  is  the  only  important  stream 
entering  the  Ohio  between  Cincinnati  and  Louisville. 

By  taking  the  heights  of  crest  waves  at  Cincinnati,  and  the  corre- 
sponding heights  at  Louisville,  arranging  them  according  to  the  magni- 
tude of  the  stage  at  Cincinnati,  and  taking  the  average  of  groups  from 
15  feet  to  20,  20  feet  to  25,  etc.,  and  then  interpolating  to  the  nearest 
five  feet  25,  30,  35,  etc.,  the  corresponding  crest  stages  at  the  two  places 


RIVER-STAGE  PREDICTIONS. 


231 


are  found  to  be  the  following.  The  numbers  with  plus  or  minus  prefixed 
are  the  probable  errors.  The  error  of  a  stage  will  rarely  be  greater  than 
three  times  the  probable  error. 

The  stages  of  water  at  Louisville  are  those  given  by  the  city  gauge  at 
the  foot  of  Fourth  Street. 

The  flood  line  is  at  24  feet.  The  highest  water,  that  of  February 
1 6,  1884,  was  46.6  feet. 

The  time  of  a  flood  wave  from  Cincinnati  to  Evansville,  Ind.,  is  about 
four  days.  The  distance  is  316  miles. 

The  flood  line  is  at  30  feet.  The  high  water  of  February  19,  1884, 
was  48.0  feet. 

The  time  of  a  flood  wave  from  Cincinnati  to  Mount  Vernon,  Ind., 
distance  352  miles,  is  about  four  days. 

HIGH- WATER  CRESTS. 


CINCINNATI 
RIVER  STAGE. 

LOUISVILLE. 

EvANSVILLB. 

MOUNT 

VERNON. 

Feet. 

Feet.                 Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

10 

5-2 

±0.8 

IS 

74          db  0.5 

10.0 

±1.4 

2O 

8-7             ±  0-3 

14.4 

±    -5 

25 

10.2 

19.4 

±    -4 

17.7 

±1.8 

30 

1  1.8         ±  1.0 

24.0 

±    .6 

22.5 

35 

13.8 

28.6 

±    -7 

27.7 

±2.1 

40 

16.9         ±  I.I 

32.8 

±    -7 

35-6 

45 

20.4 

37-o 

±  1.0 

40.7 

50 

26.0         ±1.4 

41.2 

±  1.2 

43-3 

55 

31.5 

44-6 

±i-7 

44-9 

60 

38.2 

47.0 

46.4 

65 

43-4 

48.0 

70 

46.4 

48.1 

The  method  of  using  the  table  is  as  follows :  For  a  stage  of  50  feet 
at  Cincinnati,  the  stage  to  be  expected  one  day  after  at  Louisville  is 
26.0  feet,  at  Evansville  four  days  after  41.2,  and  at  Mount  Vernon  43.3. 
When  the  exact  value  of  the  Cincinnati  stage  is  not  in  the  first  column, 


232 


METEOROLOGY. 


a  proportional  part  of  the  differences  of  corresponding  crests  is  to  be 
used  in  finding  the  computed  crests.  The  stage  at  Cincinnati  being 
53  feet,  for  instance,  the  stage  at  Evansville  will  be  three-fifths  of  the 
way  from  41.2  to  44.6,  or  43.2  feet. 

Ohio  River,  Cairo,  I1L  —  For   predicting  river  stages  at  Cairo  six 
days  ahead,  the  stages  used  are  observed  at  St.  Louis  on  the  Mississippi 


River,  at  Cincinnati  on  the  Ohio,  at  Nashville  on  the  Cumberland,  and 
at  Chattanooga  on  the  Tennessee.  These  places  are  shown  on  the 
map.  Besides  the  water  passing  these  places,  large  contributions  of 
water  are  received  by  the  Ohio  from  the  Wabash,  the  Kentucky,  and 


RIVER-STAGE  PREDICTIONS. 


233 


the  Green  rivers.  Besides  the  water  passing  the  places  mentioned, 
there  is  water  passing  Cairo  which  drains  from  108,000  square  miles 
of  additional  drainage  area  below  the  places. 

The  distance  from  Cairo  to  St.  Louis  is  200  miles ;  to  Cincinnati, 
499  miles;  to  Nashville,  227  miles;  to  Chattanooga,  488  miles.  A 
high-water  crest  at  Cairo  usually  follows  in  from  six  to  eight  days  after 
a  crest  at  Cincinnati.  In  76  cases  the  intervals  between  crests  were  as 
follows :  3  days  in  I  case,  4  days  in  4  cases,  5  days  in  5  cases,  6  days  in 
41  cases,  7  days  in  9  cases,  8  days  in  5  cases,  9  days  in  3  cases,  10  days 
in  4  cases,  n  days  in  2  cases,  12  days  in  i  case,  and  14  days  in  i  case. 

The  six-day  rise  at  Cairo,  multiplied  by  the  stage  at  Cairo  oh  the  day 
of  the  Cincinnati  crest,  being  placed  equal  to  the  sum  of  the  products 
of  the  preceding  six-day  rise  at  Cincinnati,  the  three-day  rise  at  Nash- 
ville, the  six-day  rise  at  Chattanooga,  and  the  four-day  rise  at  St.  Louis, 
by  the  mean  stages  at  the  places,  and  each  product  by  an  unknown  fac- 
tor, and  being  grouped  according  to  the  magnitude  of  the  Cairo  stage, 
the  following  factors  were  derived  for  the  various  places  and  for  Cairo 
as  dependent  on  the  stage :  for  Cincinnati,  0.69 ;  for  Nashville,  0.62 ; 
for  Chattanooga,  0.42 ;  and  for  St.  Louis,  0.32. 


CAIRO 
RIVER  STAGE. 

FACTOR. 

CAIRO 
RIVER  STAGE. 

FACTOR. 

CAIRO 
RIVER  STAGE. 

FACTOR. 

Feet. 

Feet. 

Feet. 

2O 

0-353 

30 

0.308 

40 

0.237 

21 

0.352 

31 

0.304 

41 

0.227 

22 

0-347 

32 

0.300 

42 

0.216 

23 

0-343 

33 

0.294 

43 

0.204 

24 

0-337 

34 

0.287 

44 

0.194 

25 

0.334 

35 

0.283 

45 

0.182 

26 

0.329 

36 

0.276 

46 

0.171 

27 

0.324 

37 

0.270 

47 

0.159 

28 

0.319 

38 

0.258 

48 

0.150 

29 

0.314 

39 

0.248 

The  rule  for  deriving  the  Cairo  rise  is  as  follows :  When  the  river  has 
been  rising  at  Cincinnati  for  at  least  six  days,  and  has  reached  a  crest, 
then  the  rise  at  Cairo  in  the  next  six  days  will  be  equal  to  the  preceding 


234 


METEOROLOGY. 


six-day  rise  at  Cincinnati  multiplied  by  the  mean  stage  of  water  at  Cin- 
cinnati on  the  day  of  the  crest,  and  six  days  before,  and  by  the  factor 
0.69,  plus  the  three-day  rise  at  Nashville  multiplied  by  the  mean  stage 
and  the  factor  0.62,  plus  the  six-day  rise  at  Chattanooga  multiplied  by 
the  mean  stage  and  the  factor  0.42,  plus  the  four-day  rise  at  St.  Louis 
multiplied  by  the  mean  stage  and  the  factor  0.32,  the  whole  divided  by 
the  stage  at  Cairo  and  multiplied  by  the  factor  given  on  page  233  for 
the  Cairo  stage. 

The  following  is  an  example  of  the  method  of  computation :  March  i, 
1890,  the  stage  of  water  at  Cincinnati  was  56.8  feet  and  at  a  crest ;  on 
February  23  the  stage  was  43.0  feet.  At  Nashville  the  stage  on  March  i 
was  47.2  feet,  and  on  February  26,  37.3  feet.  At  Chattanooga  the  stage 
on  March  I  was  40.2  feet,  and  on  February  23,  7.2.  At  St.  Louis  the 
stage  on  March  I  was  8.6,  and  on  February  25,  8.4.  The  stage  at  Cairo 
on  March  i  was  42.1.  Hence  the  rise  to  be  expected  at  Cairo  in  the 
next  six  days  according  to  the  rule  was :  — 

0.204  [(13.8x50x0.69) 
4-  (9.9x42x0.62) 
+  (33.0x24x0.42) 
—  (0.2  x  8x0.32)] -1-42  =  5. 2. 

The  observed  rise  by  March  7  was  5.1  feet.     The  crest,  however,  did 
not  occur  until  March  n,  when  there  was  an  additional  rise  of  1.6  feet. 
The  following  are  some  of  the  important  rises  at  Cairo  computed 
according  to  the  rule  :  — 


DATS. 

CAIRO 
RIVER  STAGE. 

OBSERVED  RISE 
IN  Six  DAYS. 

COMPUTED 
RISE. 

ERROR. 

April  17  l88l  

Feet. 
448 

Feet. 
O  7 

Feet. 
I  i 

Feet. 
4-  08 

February  21,  1882.  .  .  . 
February  14,  1884  .... 
April  9,  1886  

474 
48.2 

47.8 

V'J 
4-3 
3-3 

2  d 

i-9 
1.8 

—  Oul 

-2.4 

-*•$ 

—  2.8 

March  31,  1888  .... 
January  21,  1890  .... 

43-8 
43-7 

42.1 

14 
—  I.I 
5.1 

2.6 

1.8 
c.6 

+  1.2 
+  2.9 

+  o.c 

46.7 

1.8 

2.2 

f-o.4 

RIVER-STAGE  PREDICTIONS. 


235 


Cairo  Rise,  Three  Days  Ahead. — The  rises  at  Cairo,  111.,  can  also  be 
estimated  three  days  ahead  from  the  rises  at  St.  Louis,  Mo.,  on  the 
Mississippi ;  Mount  Carmel,  111.,  on  the  Wabash  ;  Evansville,  Ind.,  on  the 
Ohio  ;  Nashville,  Tenn.,  on  the  Cumberland  ;  and  Johnsonville,  Tenn., 
on  the  Tennessee ;  all  about  the  same  distance  by  river  above  Cairo, 
111.  A  rise  at  any  of  these  places  has  its  full  effect  three  days  after  in 
producing  a  rise  at  Cairo  in  the  same  time.  The  following  tables  give 
the  relations  of  the  rises  at  these  places  to  the  subsequent  rises  at 
Cairo,  as  deduced  from  the  cross-sections,  discharges,  and  slopes  of  the 
river  at  the  various  places  throughout  all  the  stages,  at  the  various 
places  taken  in  connection  with  the  rises  as  observed  at  some  of  the 
stages.  Corresponding  rises  have  not  been  observed  at  all  the  stages. 

The  Wabash  River  comes  into  the  Ohio  below  Evansville,  Ind.  The 
effect  of  a  rise  at  Mount  Carmel,  111.,  on  the  Wabash,  is  estimated  most 
conveniently  in  terms  of  the  rise  at  Evansville.  In  using  the  tables, 
therefore,  a  rise  or  fall  at  Mount  Carmel  should  be  applied  to  the  rise  or 
fall  at  Evansville  during  the  same  time,  and  then  with  the  Evansville 
rise  the  effect  estimated  on  Cairo  by  means  of  the  Evansville  table. 

The  Nashville  three-day  rise  in  the  Cumberland  River,  divided  by 
subsequent  three-day  rise  at  Cairo,  equals  the  following  fractions. 
Nashville,  2 1 5  miles  above  Cairo. 


CAIRO  STAGE.  FT. 

5 

10 

15 

2O 

25 

30 

35 

40 

45 

50 

NASHVILLE  STAGE.  FT. 

5 

.16 

.16 

.14 

.11 

.08 

.07 

.06 

.04 

.04 

.04 

10 

.21 

.18 

.18 

•13 

.09 

.07 

.07 

.06 

.06 

.04 

'5 

•25 

•23 

.21 

•17 

.11 

.08 

.07 

.07 

.06 

.06 

20 

.28 

•25 

•25 

.19 

.12 

.11 

.07 

.07 

.07 

.06 

25 

.30 

.28 

.28 

.21 

.14 

•13 

.08 

.10 

.08 

.07 

3° 

.32 

•30 

•30 

•23 

•15 

•13 

.10 

.10 

.08 

.08 

35 

•35 

•32 

•30 

•25 

.18 

.14 

.11 

.10 

.10 

.08 

40 

.46 

.41 

.41 

•34 

•23 

.18 

.14 

.14 

•13 

•13 

45 

.48 

.46 

.41 

•34 

.24 

.21 

•'5 

.15 

•13 

•*3 

236 


METEOROLOGY. 


The  Evansville,  Ind.,  three-day  rise  in  the  Ohio  River,  divided  by  sub- 
sequent three-day  rise  at  Cairo,  equals  the  following  fractions.  Evans- 
ville, 183  miles  above  Cairo. 


CAIRO  STAGE. 

5 

10 

15 

20 

25 

30 

35 

40 

45 

5° 

EVANSVILLE  STACK. 

5 

•23 

.21 

.18 

•17 

.14 

.12 

.09 

.07 

.06 

•°5 

10 

•33 

.29 

.26 

.24 

.20 

.17 

.14 

.10 

.09 

.07 

'5 

.41 

.36 

•34 

•31 

.26 

.25 

.20 

•15 

•13 

.10 

20 

•5i 

•45 

•44 

•38 

•33 

.28 

•25 

.21 

.16 

.14 

*5 

.64 

•57 

.56 

•50 

.41 

•36 

.28 

.24 

.19 

.18 

30 

.65 

.64 

•57 

•56 

.48 

•37 

.36 

.28 

•24 

.21 

35 

•74 

.65 

.64 

•63 

•5° 

43 

•37 

•33 

.28 

.22 

40 

•75 

•74 

.65 

.64 

.56 

49 

43 

•37 

•30 

.26 

45 

.98 

.86 

.84 

•83 

•74 

•63 

•54 

.48 

.40 

•35 

The  St.  Louis,  Mo.,  three-day  rise  in  the  Mississippi  River,  divided 
by  the  subsequent  three-day  rise  at  Cairo,  equals  the  following  frac- 
tions. St.  Louis,  1 68  miles  above  Cairo. 


CAIRO  STAGE. 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

ST.  Louis  STAGE. 

5 

.78 

•7' 

•57 

•36 

•31 

•25 

.20 

•15 

•13 

10 

•87 

.87 

.65 

•44 

•36 

.29 

.22 

.19 

•15 

»5 

1.  00 

.80 

.66 

•53 

45 

•36 

.28 

.22 

20 

1.03 

.87 

•7i 

.60 

49 

•36 

•30 

25 

•73 

.66 

.58 

.41 

•34 

30 

•78 

•75 

.68 

•52 

•44 

35 

•77 

•54 

.46 

40 

.78 

.60 

•5i 

RIVER-S  TA  GE  PR  EDI C  TIONS. 


237 


The  Johnsonville,  Term.,  three-day  rise  in  the  Tennessee  River, 
divided  by  the  subsequent  three-day  rise  at  Cairo,  equals  the  following 
fractions.  Johnsonville,  140  miles  above  Cairo. 


CAIRO  STAGE. 

5 

10 

15 

20 

25 

30 

35 

40 

45 

50 

JOHNSONVILLE  STAGE. 

5 

•39 

•35 

•30 

.24 

•'5 

.11 

.08 

.07 

.04 

.04 

10 

.46 

•44 

•39 

•30 

.20 

•15 

.11 

.08 

.07 

.06 

'5 

.48 

•44 

•34 

•23 

•17 

•13 

.11 

.10 

.07 

20 

•53 

.38 

.26 

.21 

•17 

•13 

.11 

.10 

25 

•44 

•30 

.22 

.18 

•i5 

.13 

.11 

30 

•35 

.27 

.24 

.18 

•17 

.14 

35 

•31 

.24 

.21 

.20 

•15 

40 

.28 

•25 

.21 

.18 

45 

.27 

.24 

.21 

The  Mount  Carmel,  111.,  rise  in  the  Wabash  River,  divided  by  the 
corresponding  rise  in  the  Ohio,  as  measured  by  gauge  at  Evansville, 
Ind.  equals  the  following  fractions.  Mount  Carmel,  178  miles  above 
Cairo. 


EVANSVILLE  STAGE. 

5 

10 

15 

20 

25 

3° 

35 

40 

45 

MOUNT  CARMEL  STAGE. 

5 

.61 

•52 

•51 

48 

.46 

•44 

•44 

.44 

•37 

10 

.72 

.62 

•59 

•57 

•54 

•52 

•52 

•52 

•43 

15 

•78 

.67 

.64 

.61 

.58 

.56 

.56 

.56 

•47 

20 

1.05 

•91 

.86 

•83 

•79 

.76 

.76 

.76 

•63 

25 

1.44 

1.24 

1.18 

1.  12 

1.08 

1.04 

1.04 

1.04 

•87 

30 

1.56 

'•33 

1.28 

1.22 

1.17 

1.  12 

1.  12 

1.  12 

•93 

The  rise  at  Cairo  in  three  days  is  the  sum  of  the  rises  at  the  various 
places  in  the  preceding  three  days  after  being  multiplied  by  the  frac- 
tions given  in  the  tables.  This  always  gives  a  very  close  approximation 
to  the  Cairo  stage  three  days  ahead.  In  deriving  the  rule,  some  dis- 
tinction should  have  been  made  between  the  rises  occurring  suddenly 
and  those  that  occur  more  slowly.  A  rise  that  takes  place,  for  instance, 
in  one  day,  and  persists  at  the  high  stage  for  the  next  two  days,  has  a 


238 


METEOROLOGY. 


greater  effect  in  producing  a  rise  at  points  below  it  than  the  same  rise 
extending  over  three  days.  For  the  important  factors,  however,  in 
producing  a  rise  at  Cairo,  St.  Louis,  and  Evansville,  the  volumes  of 
water  for  high  stages  are  great,  and  the  rises  are  slow  and  gradual. 

The  drainage  area  above  St.  Louis  is  699,000  square  miles;  above 
Mount  Carmel,  26,300;  above  Evansville,  99,700;  above  Nashville, 
1 1, 600;  and  above  Johnsonville,  36,700.  The  water  from  additional 
drainage  area  passing  Cairo,  but  not  passing  those  places,  is  37,600 
square  miles. 

Cumberland  River,  Carthage,  Tenn. ;  Nashville,  Tenn. ;  and  Eddyville, 
Ky.  —  The  time  from  a  high-water  crest  at  Burnside,  Ky.,  on  the  Cum- 
berland River,  to  Carthage,  Tenn.,  177  miles,  is  two  days ;  to  Nashville, 
259  miles,  three  days ;  to  Eddyville,  394  miles,  five  days.  The  compara- 
tive crests  at  these  places  are  as  follows  :  — 

HIGH-WATER  CRESTS,  CUMBERLAND  RIVER. 


BURNSIDE. 

CARTHAGE. 

NASHVILLE. 

CARTHAGE. 

NASHVILLE. 

NASHVILLE. 

EDDYVILLE. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

10 

9.9  ±  2.2 

II-3  ±1-7 

10 

I3-I  ±  LI 

10 

10.0  ±  1.8 

15 

14.0  ±  2.4 

16.5  ±  2.3 

15 

17.9  ±  1.4 

15 

I6.7 

20 

17.6  ±  1.6 

20.5  ±  14 

20 

21.0  ±  2.3 

20 

23.0 

25 

21.6  ±  2.2 

24.8 

25 

27.3  db  1.8 

25 

29.4  ±  1.8 

3° 

25.8 

3M  ±  o-5 

30 

33-4  ±  2.8 

30 

36.7 

35 

28.3  ±  2.2 

36.6 

35 

36.9  ±  1.8 

35 

44-2 

40 

31-5  ±  2.3 

39-9 

40 

41.0  ±  0.8 

40 

49.8 

45 

35.0  ±  0.2 

42.6 

45 

45 

53-0 

50 

39-1  ±  o-5 

44.6 

50 

5o 

55-5 

55 

44.0 

46.5 

60.3 

60 

48.4 

48.6 

62 

50.0 

494 

74 

64 

55-2 

The  flood  line  at  Burnside  is  at  30  feet.  The  stage  has  been  as  high 
as  74  feet. 

The  flood  line  at  Carthage  is  at  30  feet.  The  water  sometimes  rises 
as  high  as  64  feet. 


RIVER-STAGE  PREDICTIONS. 


239 


The  flood  line  at  Nashville  is  at  40  feet.  The  highest  recorded  stage 
of  water  is  55.2  feet. 

The  flood  line  at  Eddyville  is  at  31  feet.  The  highest  water,  60.3  feet, 
occurred  in  1882. 

Tennessee  River,  Chattanooga,  Tenn. ;  Decatur,  Ala. ;  Florence,  Ala. ; 
and  Johnsonville,  Tenn.  —  The  stages  of  water  at  Chattanooga  are  the 
result  of  the  stages  two  days  preceding  at  Knoxville  on  the  Tennessee, 
158  miles  above,  and  at  Clinton  on  the  Clinch  River,  148  miles  above. 
Some  water  is  also  added  to  the  Tennessee  by  the  Hiawassee,  which 
enters  the  river  35  miles  above  Chattanooga. 

When  the  river  is  rising  at  Chattanooga  the  approximate  rise  two 
days  after  a  crest  at  Clinton  or  Knoxville  is  obtained  as  follows : 
Multiply  the  two-day  rise  at  each  of  the  places  by  the  mean  stage  and 
the  sum  of  the  products  by  the  factor  given  below,  and  divide  by  the 
stage  at  Chattanooga. 


CHATTANOOGA 
RIVER  STAGE. 

FACTOR. 

CHATTANOOGA 
RIVER  STAGE. 

FACTOR. 

Feet. 

Feet. 

10 

0.90 

35 

0.28 

IS 

0.82 

40 

0.24 

20 

0.67 

45 

0.21 

25 

o-45 

50 

0.18 

30 

o-35 

EXAMPLE.  —  At  Clinton,  February  26  to  28,  1890,  the  river  rose  from 
a  stage  of  23.8  to  35.5  feet,  and  at  Knoxville  from  12.9  to  23.0.  The 
stage  at  Chattanooga  on  February  28  was  34.8.  Hence  the  rise  at 
Chattanooga  by  March  2d,  according  to  the  rule,  was :  — 

0.28(11.7x30+10.1  x  18) 


35 


4.3. 


The  rise  observed  was  7.7  feet. 

The  wave-crest  time  from  Chattanooga,  Tenn.,  to  Decatur,  Ala.,  160 
miles,  is  two  days  ;  to  Florence,  Ala.,  208  miles,  three  days ;  to  John- 
sonville, Tenn.,  360  miles,  five  days.  A  large  volume  of  water  is  added 
to  the  Tennessee  by  the  Duck  River  at  a  short  distance  above  Johnson- 


240 


METEOROLOGY. 


ville,  which  renders  computed  crests  for  that  place  less  accurate  than 
for  points  farther  up  the  river.  The  record  at  Columbia,  on  the  Duck 
River,  has  not  yet  been  kept  long  enough  to  make  the  stages  of  use  for 
lower  points. 

In  the  following  table  are  given  the  comparative  crest  for  the  various 
places  on  the  Tennessee  River :  — 

HIGH-WATER  CRESTS,  TENNESSEE  RIVER. 


CLINTON. 

CHATTANOOGA. 

CHATTANOOGA. 

DECATUR. 

FLORENCE. 

JOHNSONVILLE. 

10 

12.0 

10 

'5 

17.0 

15 

12.2  ±  0.7 

1  1.  1  ±  0.8 

20 

21.6 

20 

14-4  ±  0-9 

13-8  ±  1-3 

25 

27-3 

25 

I6.5 

16.6 

30 

34-o 

30 

18.5  ±  0.8 

19.6  ±  2.3 

35 

41-5 

35 

20.4 

22.4 

33-6 

40 

47-3 

40 

22.7 

26.4 

37-i 

45 

52.2 

45 

24.6 

27.6 

39-9 

50 

26.0 

27.8 

41.4 

52 

26.7 

28.0 

41.8 

The  flood  line  at  Clinton  is  at  25  feet. 

The  flood  line  at  Chattanooga  is  at  33  feet.  The  highest  water,  58.0 
feet,  occurred  March  n,  1867. 

The  flood  line  at  Decatur  is  at  21  feet. 

The  flood  line  at  Johnsonville  is  at  21  feet. 

Missouri  River  and  Upper  Mississippi  River.  —  High-water  crests  in 
the  Missouri  River  pass  Omaha,  Neb.,  one  day  and  a  half  later  than  at 
Sioux  City,  135  miles  above. 

The  flood  line  at  Omaha  is  at  18  feet. 

High-water  crests  pass  Kansas  City  two  and  a  half  days  after  Omaha, 
282  miles  above. 

The  flood  line  at  Kansas  City  is  at  21  feet.  The  high  water  of  1844 
was  37  feet. 

The  flood-wave  time  from  Kansas  City  to  Jefferson  City,  Mo.,  240 
miles  below,  is  three  days. 


RIVER-STAGE  PREDICTIONS. 


24I 


The  flood  line  at  Jefferson  City  is  at  20  feet.  The  high  water  of 
1844  was  28.5  feet. 

High-water  crests  in  the  upper  Mississippi  River  pass  from  Dubuque, 
la.,  to  Davenport,  la.,  a  distance  of  99  miles,  in  one  day. 

The  flood  line  at  Davenport  is  at  15  feet.  The  high  water  of  1868 
was  20.9  feet. 

The  comparative  crests  at  these  places  are  shown  in  the  following 
table :  — 

HIGH- WATER  CRESTS,  MISSOURI  RIVER  AND  UPPER  MISSISSIPPI. 


DUBUQUE. 

DAVENPORT. 

Sioux 
CITY. 

OMAHA. 

OMAHA. 

KANSAS 
CITY. 

KANSAS 
CITY. 

JEFFERSON 
CITY. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

5 

4.  2  ±0.8 

6 

4-7 

7 

5-3 

8 

5-  8  ±0.7 

8 

9.0 

9 

6.7 

9 

9-8 

10 

7.6  ±  0.6 

10 

10.8  ±  0.8 

IO 

14.8  ±  1.4 

II 

8.6 

II 

11.8 

II 

'5-3 

12 

9-7 

12 

12.8 

12 

15.8 

13 

10.5 

13 

13.8 

'3 

16.6 

13 

10.3 

H 

"•3 

14 

14.6 

14 

'7-3 

14 

II.  O 

IS 

12.0  ±  0.3 

15 

15.4  ±  0.6 

'5 

18.1  ±  1.2 

15 

"•7±i-3 

16 

12.7 

16 

1  6.0 

16 

18.7 

16 

12.  3  ±0.6 

17 

13-6 

*7 

'7-5 

'7 

19.4 

17 

13-2 

It 

14-5 

18 

19.0 

18 

20.4 

18 

13-9 

19 

15.5 

19 

20.5 

19 

21.4 

19 

14.7 

20 

I6.4 

20 

22.  0 

20 

22.4 

20 

15-5 

21 

17-3 

21 

23.2 

21 

23-3 

21 

16.3 

22 

23.8 

22 

24-3 

22 

17.0 

23 

25-3 

23 

17.6 

24 

26.3 

24 

18.0 

25 

18.4 

26 

18.7 

242  ME  TE  OR  0  LOG  Y. 

Missouri  River  Flood.  —  The  great  flood  along  the  lower  Missouri 
River  in  1881  was  preceded  by  a  very  severe  winter.  The  ice  at  the 
mouth  of  the  river  was  2  to  3  feet  thick,  and  at  Sioux  City  and  Ver- 
million,  4  to  5  feet.  Warm  weather  set  in  on  the  eastern  slopes  of  the 
Rocky  Mountains  in  February,  while  it  was  still  cold  in  the  East.  The 
ice  was  tumbled  east  by  the  floods,  and  the  rivers  gorged  and  the  bar- 
riers across  the  valleys  flooded  the  country  back  of  them.  The  lower 
rivers  were  at  a  low  stage  when  the  great  flood  came  after  April  17, 
entirely  from  above  Sioux  City,  la.,  and  was  all  snow  water.  The 
valley  between  the  bluffs,  averaging  5  miles  in  width  for  376  miles  from 
Sioux  City,  la.,  to  Glascow,  Mo.,  was  submerged.  Much  of  the  water 
did  not  pass  the  gauge  at  Omaha,  but  passed  around  it  through  sloughs 
and  low  places.  The  river  left  its  bed  and  went  down  the  centre  of  the 
valley,  leaving  deposits  6  to  12  feet  in  depth  in  various  places.  After 
the  river  got  out  of  its  banks  from  Omaha  to  Kansas  City,  the  velocity 
slackened  and  the  rate  of  rise  increased.  In  a  few  hours  it  slackened 
so  skiffs  could  go  about  with  ease,  although  before  it  left  its  banks,  and 
after  its  return,  the  current  was  so  swift  that  steamboats  could  not 
stem  it. 

Contrary  to  what  usually  happens,  the  crest  of  the  flood  wave  was 
greatly  retarded.  The  front  lengthened  out,  while  the  rear  shortened 
up.  At  the  same  time  the  total  length,  from  hollow  to  hollow,  remained 
nearly  constant,  but  the  crest  was  shifted  from  front  to  rear  in  its 
progress  down  the  valley. 

Ordinarily,  gauge  readings  will  show  with  unobstructed  condition  of 
flow  as  the  surface  rises  the  sectional  area,  velocity  and  discharge 
increase  at  a  rate  proportional  to  the  increase  of  gauge  reading.  As 
the  water  surface  rises  the  velocity  increases,  and  the  mass  of  water 
moving  forward  more  rapidly  tends  to  overtake  and  run  upon  that 
which  precedes  it,  so  that  the  crest  advances  more  rapidly  than  the 
front  of  the  wave,  and  the  front  slope  steepens.  The  reverse  occurs 
on  the  rear  slope.  Crests  usually  take  6£  days  from  Sioux  City  to  St. 
Charles  at  the  mouth.  That  of  1881  took  12  days,  and  was  7  feet 
higher  than  any  previous  one  at  Sioux  City. 

The  heights  reached  by  the  flood  of  1881  were  abnormal  for  the 
quantity  of  water  passing  solely,  because  the  river  was  out  of  its  banks 


RIVER-STAGE  PREDICTIONS.  243 

and  the  progress  of  the  water  was  so  retarded  by  frictional  resistance 
being  transferred  from  its  normal  section  to  one  of  small  depth  and 
miles  in  width.  The  slope  by  this  was  doubled,  but  not  enough  to  off- 
set increased  resistance,  and  water  in  rear  piled  up  on  that  in  front, 
causing  an  abnormal  rise  at  St.  Charles,  of  5  to  8  feet. 

Mississippi  River  at  St.  Louis,  Mo.  —  A  rise  of  the  Mississippi  at  St. 
Louis,  Mo.,  amounting  to  several  feet,  can  be  estimated  with  some 
accuracy  four  days  ahead  of  its  occurrence.  The  basis  of  the  estimates 
are  the  river-gauge  readings  at  St.  Louis,  and  the  readings  of  the 
gauges  at  places  above  it  on  the  Missouri  and  Mississippi  rivers,  and 
the  observations  of  rainfall  in  the  drainage  area  of  130,000  square  miles 
immediately  above  St.  Louis.  The  back  records  of  stages  during  great 
rises,  and  the  observations  of  rainfall,  furnish  the  means  of  deriving  a 
rule  for  estimating  the  stages.  The  record  of  stages  at  St.  Louis 
begins  in  1861. 

The  situation  of  the  various  river  gauges  above  St.  Louis  are  shown 
on  the  accompanying  map. 

The  principal  rise  of  the  river,  which  occurs  in  May  or  June,  and  is 
popularly  known  as  the  June  rise,  is  due  to  rainfall  in  the  lower  part 
of  the  drainage  area  mainly,  and  not  to  a  rise  from  the  upper  Missouri 
River  and  the  melting  snow  in  the  mountains,  as  commonly  sup- 
posed. 

The  highest  water  of  which  the  record  is  certain  was  41.4  feet  on 
June  27,  1844.  There  is  said  to  have  been  a  stage  of  42.0  feet  in 
April,  1785.  There  was  a  stage  of  33.6  feet  in  March,  1828  ;  32.4  feet, 
June  27,  1845  ;  30.0  feet,  May  7,  1853  ;  37.0  feet,  June  14,  1858  ;  31.4 
feet,  April  26,  1862;  32  feet,  May  10,  1876;  33.6  feet,  May  5,  1881  ; 
32.2  feet,  July  5,  1882  ;  34.8  feet,  June  26,  1883  ;  36  feet,  May  19,  1892  ; 
and  31.5  feet,  May  3,  1893. 

The  average  of  the  annual  highest  water  is  26.3  feet,  the  lowest 
being  18.0  feet  in  1863.  The  height  of  30  feet  has  been  exceeded 
seven  years  of  the  thirty-three  years  during  which  time  there  are 
records. 

The  average  date  of  highest  water  of  the  year  is  May  27 ;  the  earliest, 
March  4 ;  the  latest,  July  28. 

The  greatest  number  of  days  above  the  danger  line  in  a  year,  the 


244 


METEOROLOGY. 


3    V, 


RIVER-STAGE  PREDICTIONS.  24$ 

stage  of  30  feet,  were  36  days  in  1892,  17  in  1883,  12  in  1 88 1,  7  in  1876, 
and  9  in  1862. 

The  average  lowest  water  of  the  year  is  3.0  feet,  and  the  average 
time  of  its  occurrence,  December  27.  The  greatest  number  of  days 
below  a  stage  of  3  feet  was  33  in  1864.  The  extreme  lowest  water, 
o.o  feet  at  the  zero  of  the  gauge,  occurred  December  21,  1863. 

The  annual  stage  at  St.  Louis  from  32  years'  observations  is  12.97 
feet.  The  ten-year  means  show  a  slight  progressive  increase,  being 
12.30  feet  for  1860-1870,  12.61  for  1870-1880,  and  14.0  for  1880-1890. 

Although  the  water  that  flows  by  Kansas  City  and  Dubuque  com- 
prises the  drainage  from  568,900  square  miles  of  the  total  drainage 
area  above  St.  Louis,  yet  the  rises  of  the  rivers  at  those  places  have  a 
relatively  small  effect  in  producing  rises  subsequently  at  St.  Louis  and 
the  river  stages  at  the  places  are  not  closely  dependent.  This  is  due  to 
the  great  rainfall  in  the  drainage  area  of  130,000  square  miles  immedi- 
ately above  St.  Louis  being  so  much  greater  than  in  the  part  of  the 
drainage  basin  higher  up. 

The  drainage  area  above  Kansas  City,  Mo.,  is  491,800  square  miles  ; 
the  distance  to  St.  Louis  is  401  miles;  the  zero  of  the  gauge  is  337.2 
feet  higher  than  the  zero  of  the  gauge  at  St.  Louis.  The  drainage  area 
above  Dubuque,  la.,  is  77,100  square  miles ;  the  distance  to  St.  Louis  is 
437  miles  ;  the  zero  of  the  gauge  is  104.3  ^eet  above  the  zero  of  the 
gauge  at  St.  Louis. 

The  time  of  water  travel  from  Kansas  City  and  Dubuque  to  St. 
Louis  is  four  to  five  days. 

The  rises  to  high  water  at  Kansas  City  and  Dubuque  are  very  slow 
and  regular. 

At  Kansas  City,  in  the  cases  of  crests  to  16  feet,  the  average  daily 
rises  are :  — 

Days  ...  6  to  5  5  to  4  4  to  3  3  to  2  2  to  i  i  to  crest 
Feet  ...  o.o  0.6  i.o  i.i  0.8  0.7 

and  for  crest  above  16  and  to  26  feet,  they  are  :  — 

Days  ...  6  to  5  5  to  4  4  to  3  3  to  2  2  to  i  i  to  crest 
Feet  .  .  .  0.3  0.4  0.4  0.9  0.8  0.5 


246  METEOROLOGY. 

At  Dubuque,  in  cases  of  crests  to  20  feet,  the  average  daily  rises 
are:  — 

Days      ...     6  to  5         5  to  4         4  to  3         3  to  2         2  to  i          i  to  crest 
Feet      ...      0.6  0.5  0.5  0.4  0.5  0.5 

The  highest  water  at  Kansas  City,  37  feet,  occurred  June  20,  1844. 
The  next  highest  was  26.3  feet,  April  30,  1881.  The  greatest  rise  in  a 
day  is  rarely  as  much  as  4  feet. 

The  highest  water  at  Dubuque  was  22.7  feet,  June  24,  1880.  The 
greatest  rise  in  a  day  is  rarely  as  much  as  3  feet. 

The  rise  of  river  at  St.  Louis  is  usually  slow  and  regular.  The  aver- 
age rises  for  six  days  before  crests  are  :  — 

Days     ...      6  to  5         5  to  4        4  to  3         3  to  2         2  to  i          i  to  crest 
Feet     .     .     .        0.3  0.4  0.6  0.9  i.o  0.5 

Some  of  the  greatest  rises  in  a  day  have  been,  — 

February  9  to  10,  1881,  when  there  was  a  rise  of  6.8  feet  in  the  river 
at  St.  Louis,  from  the  stage  of  10.9  to  17.7.  The  day  before  there  was 
a  rise  of  1.7,  and  the  day  after  a  rise  of  0.3.  February  6  and  7  the 
rainfall  was  1.24  inches  at  Des  Moines,  la.,  2.12  at  Leavenworth,  Kan., 
2.28  at  Boonville,  Mo.,  and  1.41  at  Hermann,  Mo. 

February  20  to  21,  1882,  there  was  a  rise  of  9.3  feet  at  St..  Louis,  from 
the  stage  of  18.2  to  27.5  ;  the  day  before  there  was  a  rise  of  7.0  feet, 
and  the  day  after  a  rise  of  0.7  of  a  foot.  The  rainfalls,  February  19  and 
20,  were:  0.95  of  an  inch  at  Keokuk,  5.12  at  Boonville,  4.93  at  Her- 
mann, 4.00  at  Jefferson  City,  and  6.71  at  St.  Louis. 

February  15  to  16,  1883,  there  was  a  rise  of  9.7  feet,  from  a  stage  of 
10.3  to  20.0  feet ;  the  day  before  there  was  a  rise  of  2.9,  and  the  i8th  an 
additional  rise  of  5.8  feet.  February  16,  the  rainfall  at  Davenport  was 
3.90  inches;  at  Keokuk,  3.56;  at  St.  Louis,  1.06  ;  at  Chicago,  1.94. 
The  main  reliance  in  predicting  a  rise  at  St.  Louis  must  be  the  use  of 
observed  heavy  rainfalls  above  it. 

Only  the  great  rises  of  river  at  St.  Louis,  due  to  heavy  rainfalls,  can 
be  estimated  with  any  accuracy.  The  method  of  estimating  a  rise  is  by 
means  of  the  heavy  rainfalls  in  the  vicinity  of  St.  Louis  and  above  it, 
using  the  back  records  of  rainfall  and  rises  to  find  the  relation  with  an 
allowance  for  the  rise  or  fall  in  the  four  days  preceding,  at  Kansas  City 


RIVER-STAGE  PREDICTIONS.  24? 

and  Dubuque.  In  this  way  the  stage  for  St.  Louis  can  be  estimated  four 
days  ahead,  with  some  accuracy  in  the  case  of  the  heaviest  rainfalls. 

In  deriving  a  rule  for  predicting  the  stage  at  St.  Louis,  it  is  neces- 
sary to  have  some  knowledge  of  the  relative  quantity  of  water  passing 
St.  Louis,  Kansas  City,  and  Dubuque,  at  different  stages,  from  low  to 
high  water.  Discharge  measurements  have  been  made  under  the  aus- 
pices of  the  Engineer  Corps  at  St.  Louis,  for  different  stages,  from  o  to 
36  feet,  and  at  Kansas  City  from  o  to  23  feet.  No  measurements  of 
discharge  have  been  made  at  Dubuque,  but  some  have  been  made  at 
Clayton,  la.,  48  miles  above  it,  for  different  stages,  from  4  to  18  feet 
above  low  water,  which  presumably  would  not  differ  very  much  from 
discharges  at  Dubuque.  Nothing  is  known  definitely  about  the  high- 
water  discharge  at  Dubuque.  It  is  safe  to  say,  however,  it  cannot  be 
as  great  as  the  flood  discharge  of  Keokuk  below  it,  which,  in  the  flood 
of  1851,  was  265,000  cubic  feet  of  water  per  second.  During  this  high 
water  the  Des  Moines  River  was  high,  and  was  pouring  out  65,000  cubic 
feet  per  second. 

The  discharge  of  a  river  is,  in  general,  not  very  closely  dependent  on 
stage  ;  that  is  to  say,  for  the  same  stage  at  different  times  there  may 
be  different  quantities  of  water  passing  in  the  river  depending  on  whether 
it  is  on  a  rising  or  a  falling  stage,  and  also  on  the  rapidity  of  the  rise  or 
fall,  the  cutting  or  filling  of  the  channel,  and  the  slope  of  the  water  sur- 
face below  and  above  the  place.  It  is  therefore  not  possible  to  assign 
a  definite  and  exact  discharge  for  each  stage.  An  approximate  estimate, 
however,  can  be  made  which  will  be  of  use  in  calculating  the  relative 
effects  of  rises  or  falls  at  Kansas  City  and  Dubuque  in  producing  a  rise 
at  St.  Louis.  The  estimated  discharges  for  stages  four  feet  apart  are 
given  in  the  following  table  :  — 


248 


METEOROLOGY. 


RIVER  DISCHARGES  IN  CUBIC  FEET  PER  SECOND. 


STACK  FEET. 

ST.  Louis. 

KANSAS  CITY. 

DUBUQUE. 

0 

48,000 

2O,OOO 

2O,OOO 

4 

8l,000 

36,000 

39.000 

8 

127,000 

62,OOO 

80,000 

12 

188,000 

107,000 

124,000 

16 

274,000 

183,000 

168,000 

20 

390.000 

285,000 

212,000 

24 

526,000 

400,000 

28 

683,000 

32 

878,000 

36 

1,146,000 

The  two  tables  following  show  the  gauge  relations  between  rises  or 
falls  at  St.  Louis,  Kansas  City,  and  Dubuque :  — 

RATIO  OF  RISES  AT  ST.  Louis  AND  KANSAS  CITY. 


ST.  Louis  STAGES. 


KANSAS   l^ITY  STAGE. 

0 

4 

8 

12 

16 

20 

24 

28 

32 

36 

0 

o-5 

0-5 

2 

0.8 

0.7 

0.5 

4 

0.9 

0.9 

0.6 

0.4 

6 

1.2 

i.i 

0.8 

0.6 

0.4 

0-3 

8 

i-5 

I.O 

0.7 

0-5 

0.4 

0-3 

10 

i.i 

0.8 

0.6 

0.4 

0.4 

0-3 

o-3 

0-3 

12 

I.O 

0.7 

0.6 

0-5 

0.4 

0-3 

0-3 

H 

1.2 

0.8 

0.7 

0.6 

o-5 

0.4 

0-3 

16 

0.9 

0.8 

0.6 

0.6 

0.4 

0-3 

18 

I.O 

0.8 

o-7 

0.6 

0.5 

0.4 

20 

0.9 

0.8 

0.7 

0.6 

0.4 

22 

0.9 

0.8 

0.7 

0.6 

0.4 

24 

0.8 

0.7 

0.6 

0.4 

RIVER-STAGE  PREDICTIONS. 

RATIO  OF  RISES  AT  ST.  Louis  AND  DUBUQUE. 


249 


DUBUQUE. 

ST.  Louis  STAGE. 

0 

4 

8 

12 

16 

20 

24 

28 

32 

36 

6  to  22 

0.8 

0.6 

0.4 

o-3 

o-3 

o-3 

0.2 

O.I 

The  method  of  estimating  the  effect  of  rainfall  in  producing  a  rise  at 
St.  Louis  was  to  select  a  number  of  cases  of  heavy  and  extensive  rain- 
fall and  compare  the  amount  with  the  subsequent  rise. 

On  the  map,  page  244,  is  shown  the  drainage  area  immediately  above 
St.  Louis  and  below  Kansas  City  and  Dubuque,  and  some  of  the  rainfall 
stations  within  the  area.  The  heavy  dotted  line  on  the  map  marks  the 
southern  and  eastern  boundary  of  the  drainage  basin  above  St.  Louis. 
The  heavy  continuous  lines  show  the  lines  of  equal  time  of  water 
travel  by  river  in  days  from  various  points  in  the  drainage  area  to  St. 
Louis,  taking  the  rate  of  travel  for  average  stages  as  3.5  miles  per  hour, 
or  84  miles  a  day. 

Of  the  rain  that  falls,  a  part  goes  directly  into  the  rivers,  and  part 
sinks  into  the  ground.  Of  this  latter,  a  part  is  fed  out  slowly  to  the 
rivers  from  springs,  but  a  greater  part  evaporates. 

Of  all  the  rain  that  falls,  the  part  of  it  that  runs  off  through  the 
rivers  is,  on  the  average,  25  to  45  per  cent  for  most  drainage  areas.  It 
varies  greatly  with  the  slope  of  the  ground,  the  hardness  of  the  soil,  and 
the  rate  of  rainfall.  In  a  heavy  rain  a  greater  proportion  of  the  water 
goes  into  the  rivers  than  in  a  light  one.  The  run-off  for  the  whole  Mis- 
souri valley  is  not,  on  the  average,  more  than  one-eighth  of  the  rainfall. 
For  the  lower  part  of  the  drainage  area,  where  the  rains  are  much 
heavier  than  in  the  upper  part,  the  run-off  must  be  much  larger. 

In  estimating  rises  at  St.  Louis,  only  the  rainfalls  greater  than  i.o 
inch  have  been  considered.  In  most  cases  the  great  rises  are  associated 
with  rains  of  2.0  to  5.0  inches  in  24  hours. 

In  the  region  of  St.  Louis,  the  rainfalls  as  regards  the  amount  of 
rainfall  in  24  hours,  is  about  as  follows,  derived  from  20  years'  observa- 
tions at  St.  Louis,  Springfield,  111.,  Kansas  City,  Des  Moines,  Keokuk, 
Davenport,  and  Dubuque.  The  table  gives  the  number  of  heavy  rain- 
falls for  any  place  in  the  region. 


250  METEOROLOGY. 

NUMBER  OF  HEAVY  24-HouR  RAINFALLS  IN  20  YEARS. 


INCHES. 

JAN. 

FEB. 

MAR. 

APR. 

MAY. 

JUNE. 

JULY. 

AUG. 

SEPT. 

OCT. 

Nov. 

DEC. 

SUMS. 

I  to  2 

5 

5 

6 

10 

18 

20 

H 

II 

14 

12 

8 

6 

I30 

2  to  3 

i 

I 

2 

4 

5 

5 

4 

4 

2 

i 

i 

30 

3  to  4 

I 

I 

I 

i 

I 

I 

I 

7 

4  to  5 

I 

I 

2 

5  to  6 

i 

I 

Of  all  the  rain  that  falls  in  the  immediate  drainage  area  above  St. 
Louis,  in  the  cases  of  rains  of  i.o  inch  or  more  in  24  hours,  it  will  be 
assumed  that  45  per  cent  of  it  reaches  the  rivers  on  the  day  of  the  fall 
and  two  days  after,  and  the  proportion  of  the  rainfall  each  day  reaching 
the  streams  will  be  taken  as  0.25  of  the  rainfall,  0.13,  and  0.07. 

In  the  cases  of  rainfall  on  successive  days  in  the  various  areas  above 
St.  Louis,  marked  by  the  lines  of  water  travel  on  the  map,  i,  2,  3,  and 
4  days  from  St.  Louis,  the  following  is  the  distribution  of  water  passing 
through  the  river  at  St.  Louis  on  successive  days  :  — 

PARTS  OF  RAINFALL  PASSING  IN  THE  RIVER  BY  ST.  Louis. 


FROM  AREA  ABOVE  ST. 
Louis. 

SUCCESSIVE  DAYS. 

DAY'S  TRAVEL. 

1 

3 

3 

4 

5 

6 

0  to  I 

•25 

•13 

.07 

I  to  2 

•25 

•13 

.07 

2  to  3 

•25 

•13 

.07 

3  to  4 

•25 

•13 

.07 

The  method  of  estimating  the  water  from  the  rainfall  which,  after 
reaching  the  rivers,  passes  St.  Louis,  is  as  follows  :  On  a  map  for  each 
day,  the  rainfall  in  inches  is  entered  that  fell  in  24  hours  at  each  place 
where  there  are  observations.  The  absence  of  any  figure  indicating 
rain  at  a  place  does  not  necessarily  mean  that  no  rain  fell.  At  the 
time  there  may  have  been  no  observations  taken. 

Blue  lines  on  the  map  are  drawn  through  the  places  having  equal 


RIVER-STAGE  PREDICTIONS.  251 

depth  of  rainfall  of  i.o  inches,  2.0  inches,  etc.  For  later  years  the 
observations  of  rainfall  at  the  numerous  voluntary  rainfall  stations  in 
existence  permit  of  forming  a  somewhat  accurate  idea  of  the  extent  of 
country  over  which  the  heavy  rainfalls  extend.  For  the  earlier  years 
previous  to  1890,  observations  of  rainfall  are  very  few. 

The  unit  of  area  in  estimating  the  quantity  of  rainfall  is  taken  as  a 
rectangle  one  degree  of  latitude  and  one  degree  of  longitude  on  a  side, 
called  a  degree  square. 

A  degree  square  comprises  an  area  of  3600  square  miles  of  the 
earth's  surface.  The  unit  of  quantity  of  water  is  taken  as  i.o  inch  over 
an  area  of  one  degree  square.  This  quantity  of  water  corresponds  to 
96,000  cubic  feet  of  water  per  second,  passing  a  place  for  one  day. 

The  extent  of  area  covered  by  different  depths  of  rainfall  can  be 
estimated  with  the  eye  with  sufficient  accuracy  for  the  purpose  in  view, 
in  terms  of  degree  squares  or  parts  of  a  square  it  covers  on  the  map. 
The  parallels  and  meridians  on  the  map  are  two  degrees  apart.  There 
is  no  need  of  resorting  to  very  accurate  measurement  of  the  areas  by  a 
planimeter,  as  it  would  involve  a  great  deal  of  labour  which  in  most  cases 
would  be  labour  wasted,  as  the  rainfall  data  is  very  incomplete,  and  the 
blue  lines  through  places  of  equal  depth  are  more  or  less  inaccurate. 

By  the  method  described,  the  quantity  of  water  flowing  from  the 
drainage  area  just  above  St.  Louis,  on  successive  days,  was  derived  for 
selected  cases  of  great  rises.  By  comparing  the  maximum  in  each 
series  with  the  corresponding  rise,  the  following  table  was  derived, 
which  gives  the  rise  at  St.  Louis  in  terms  of  the  inch-degree  squares  of 
water  computed  to  be  flowing  from  the  area. 


252 


METEOROLOGY. 


RISES  AT  ST.  Louis  IN  FEET. 


RAINFALL  IN 
INCH-DEGREE 

STAGES  AT  ST.  Louis. 

SQUARES  ABOVE 
ST.  Louis. 

6 

8 

10 

12 

14 

16 

18 

20 

22 

24 

26 

28 

30 

32 

34 

36 

0.5 

3-0 

2-5 

2-5 

2.0 

2.0 

2.O 

1-5 

'•5 

I.O 

I.O 

I.O 

I.O 

1.0 

4-5 

4.0 

3-o 

2.5 

2.O 

2.0 

2.O 

2.O 

2.O 

i-5 

I-S 

6-5 

5-5 

4-5 

3-5 

3-0 

3-o 

3-0 

3-o 

2-5 

2.0 

'•5 

I.O 

o-5 

2.0 

9.0 

7-5 

6.0 

5-o 

3-5 

3-5 

3-5 

3-5 

3-o 

2.5 

2.0 

"•5 

I.O 

2-5 

II.O 

9.0 

8.0 

6.0 

4-5 

4-5 

4.0 

4.0 

3-5 

3-o 

2-5 

2.0 

i-5 

0.7 

3-o 

13-0 

II.O 

9.0 

7.0 

5-5 

5.0 

5-o 

4-5 

4-5 

4.0 

3-o 

2-5 

2.O 

I.O 

3.5 

15.0 

13-0 

II.O 

9-5 

7-5 

7.0 

6.0 

5-5 

5-o 

5-o 

4.0 

16.5 

15.0 

13-0 

"•5 

1  0.0 

8-5 

7.0 

6.0 

6.0 

6.0 

4-5 

1  8.0 

16.5 

15.0 

13-5 

12.0 

IO.O 

8.0 

6.5 

6.0 

6.0 

5.0 

19.0 

17.0 

1  6.0 

14.0 

12.5 

10.5 

8.5 

7-0 

6.5 

6.5 

The  date  of  computed  maximum  number  of  inch-degree  squares  of 
water  passing  in  the  river  by  St.  Louis,  occurs  as  follows  in  40  cases 
that  have  been  examined  :  — 

Maximum  on  same  day  as  St.  Louis  crest,  6  times. 
Maximum  i  day  before  St.  Louis  crest,  18  times. 
Maximum  2  days  before  St.  Louis  crest,  7  times. 
Maximum  3  days  before  St.  Louis  crest,  i  time. 
Maximum  i  day  after  St.  Louis  crest,  5  times. 
Maximum  2  days  after  St.  Louis  crest,  3  times. 

In  the  table  following  the  stages  at  St.  Louis  are  given  for  the  day  of 
highest  stage,  and  for  the  three  days  preceding.  The  stages  at  Kansas 
City  and  Dubuque  are  also  given  for  four  days  preceding  the  period  of 
rise  at  St.  Louis.  The  table  gives  also  the  quantity  of  water  in  inch- 
degree  squares  computed  to  be  passing  St.  Louis  on  successive  days 
from  the  area  just  above  it. 


RIVER-STAGE  PREDICTIONS. 


253 


RIVER  STAGES  —  HEIGHTS  IN  FEET  AND  TENTHS. 


ST.  Louis. 

KANSAS  CITY. 

DUBUQUE. 

INCH-DEGREE  SQUARES 
OF  WATER. 

Date. 

Stage. 

Observed 
Rise. 

Computed 
Rise. 

Date. 

Stage. 

Date. 

Stage. 

Date. 

Number. 

26 

12.8 

26 

4.6 

1875- 

28 

I3.2 

July        30 

24.0 

30 

12.9 

30 

3-8 

30 

O.I 

31 

23.0 

31 

0.9 

Aug.         i 

25.6 

I 

1.9 

2 

28.4 

2 

3-2 

3 

29.8 

5.8 

4.6 

3 

1876. 

2 

10.9 

2 

15-4 

May        6 

25-4 

6 

I4.I 

6 

1  6.0 

6 

0.2 

7 

27.8 

8 

15-4 

7 

1.6 

8 

30.0 

8 

2.2 

9 

31-8 

9 

2-5 

10 

32.0 

6.6 

3-2 

10 

2.1 

ii 

0.9 

18 

16.2 

18 

10.8 

June      22 

24-5 

22 

16.6 

22 

1  0.0 

23 

24.0 

27 

ii'3 

24 

23-9 

25 

24.0 

25 

O.5 

26 

27.2 

2.7 

2.8 

26 

0.9 

27 

1.6 

28 

1.8 

29 

I.O 

29 

1  6.2 

29 

10.8 

2 

n.  i 

July       3 

24-3 

3 

16.3 

3 

1  1.0 

3 

4 

24-5 

5 

1  6.8 

4 

O.I 

5 

25.5 

5 

1.7 

6 

28.7 

6 

2.9 

7 

30.1 

5.8 

4-3 

7 

2.4 

8 

1.6 

9 

0.6 

1878. 

4 

7.0 

4 

3-0 

March     8 

I6.4 

8 

6.6 

8 

3-3 

8 

0.2 

9 

I5.8 

9 

6.8 

9 

1.2 

10 

17.2 

10 

2.0 

ii 

21.8 

ii 

1.8 

12 

22.8 

6.4 

3-5 

12 

1.3 

13 

0.4 

'7 

8.5 

17 

4-3 

April     21 

1  6.2 

21 

8.8 

21 

5-3 

21 

22 

1  6.8 

22 

23 

18.5 

23 

1.2 

24 

21.4 

24 

2.2 

25 

22.0 

5.8 

3-9 

11 

2.0 
I.I 

27 

0-5 

254 


METEOROLOGY. 


ST.  Louis. 

KANSAS  CITY. 

DUBUQUE. 

INCH-DEGREE 
SQUARES  OF  WATER. 

Date. 

Stage. 

Observed 
Rise. 

Computed 
Rise. 

Date. 

Stage. 

Date. 

Stage. 

Date. 

Number. 

1882. 

14 
16 

5-o 
6.0 

14 

6.0 

Feb.       1  8 

10.4 

IS 

5-7 

18 

6-3 

18 

19 

1  1.2 

19 

i-7 

20 

18.2 

20 

3-7 

21 

27-5 

21 

4-5 

22 

28.2 

I7.8 

I6.5 

22 

2.9 

23 

i-3 

1883. 

10 

4-6 

10 

Feb.       14 

74 

14 

6.0 

14 

14 

0.2 

15 

10.3 

15 

0.4 

16 

20.0 

16 

0.9 

17 

24.0 

J7 

0.8 

18 

25.8 

18.4 

4.1 

18 

0.6 

1885. 

9 

13-3 

9 

8.5 

June       13 

2O.6 

13 

16.7 

13 

7-9 

13 

0.6 

14 

22.0 

19 

19.1 

14 

1.4 

15 

24.1 

15 

'•7 

16 

26.4 

16 

1.4 

17 

27.1 

6-5 

3-2 

17 

I.O 

1886. 

21 

14.6 

21 

5-0 

June       25 

15-7 

25 

14.9 

25 

5-7 

25 

26 

154 

27 

15-2 

I 

6.1 

26 

0.4 

27 

16.2 

27 

1.2 

28 

204 

4-7 

4-7 

28 

2.2 

29 

2.6 

30 

i-7 

1889. 

May       27 

16.7 

24 

9.8 

24 

6-3 

28 

16.3 

28 

10.4 

28 

7-i 

28 

29 

16.5 

29 

10.6 

30 

74 

29 

1.2 

30 

21.8 

30 

3-o 

31 

24.4 

3i 

3-2 

June        i 

24.6 

7-9 

5-7 

i 

34 

2 

1.6 

RIVER-S  TA  GE  PREDICTIONS. 


255 


ST.  Louis. 

KANSAS  CITY. 

DUBUQUE. 

INCH-DEGREE 
SQUARES  OF  WATER. 

Date. 

Stage. 

Observed 
Rise. 

Computed 
Rise. 

Date. 

Stage. 

Date. 

Stage. 

Date. 

Number. 

1892. 

31 

13-7 

31 

4-8 

3 

0.7 

April       4 

19-3 

4 

14.0 

4 

5.6 

4 

i-3 

5 

23-4 

6 

14.8 

5 

3-i 

6 

25.1 

6 

3-6 

7 

26.5 

7 

2.6 

8 

26.8 

7-5 

6.6 

8 

i.i 

9 

O.I 

14 

154 

14 

9-3 

17 

0.4 

18 

21.8 

18 

14.4 

18 

10.5 

18 

0.4 

19 

22.5 

21 

I4.8 

19 

0.8 

20 

24.1 

20 

0.8 

21 

25-5 

21 

1.2 

22 

26.4 

4.6 

2.4 

22 

I.I 

23 

0.9 

13 

1.0 

II 

19.9 

II 

8.8 

14 

2.2 

May      15 

344 

15 

23.2 

15 

9.6 

15 

2-7 

16 

34-9 

21 

24.9 

19 

II.  2 

16 

2.7 

17 

35-3 

17 

1.9 

18 

35-6 

18 

1-3 

19 

36.0 

1.6 

0.8 

28 

I7.6 

28 

13-9 

3° 

1  6.6 

June        I 

32.3 

I 

17-5 

I 

16.9 

2 

32.6 

3 

18.1 

2 

I7.8 

3 

33-2 

3 

I.I 

4 

34-0 

4 

1.9 

5 

34-7 

2.4 

0.9 

5 

1-3 

6 

0.4 

2S6 


METEOROLOGY. 


The  following  is  an  example  of  the  method  of  estimating  a  rise  at  St. 
Louis  in  terms  of  the  rainfall  over  drainage  area  immediately  above  it, 
and  the  changes  of  stage  preceding  it  at  Kansas  City  and  Dubuque. 

On  April  4,  1892,  the  stage  at  St.  Louis  was  19.3  feet,  and  on  the  8th, 
26.8  feet ;  at  Kansas  City  the  stages  were,  March  31  and  April  4,  13.7 
and  14.0,  and  at  Dubuque,  on  the  same  dates,  4.8  and  5.6  feet.  The 
rises  at  St.  Louis  due  to  these  rises  are,  for  Kansas  City,  as  shown 
by  table  on  page  248,  0.2.  and  for  Dubuque  0.3,  and  the  sum  of  the 
two,  0.5. 

The  rainfalls  on  April  3,  4,  and  5  were  as  follows  :  — 


APRIL  3,  1892. 


INCHES. 

1.5                          3.5                         3.5 

SUMS, 

AREAS. 

INCH-DEGREE  SQUARES. 

DEGREE  SQUARES. 

0  to  I 

0.8 

o-3 

0.2 

2.7 

I  to  2 

0.4 

0.6 

2  to  3 

1.4 

0.4 

3-1 

3  to  4 

1-3 

0.3 

2.8 

4  to  5 

o-5 

0.4 

O.I 

2.2 

APRIL  4,  1892. 


INCHES. 

AREAS. 

1.5                           9.5                          3.5 

SUMS, 

DEGREE  SQUARES. 

0  to  I 

0.7 

0.6 

2.6 

I  to  2 

1.2 

1.6 

5.8 

2  to  3 

1.8 

1.2 

5-7 

3  to  4 

0.9 

1.4 

4  to  5 

0.4 

0.6 

RIVER-S  TA  GE  P RED  1C  TIONS. 


257 


APRIL  5,  1892. 


INCHES. 

AREAS. 

1.5                           3.5                           3.5 

SUMS, 

DEGREE  SQUARES. 

o  to  I 

0.2 

0.3 

I  to  2 

0.2 

o-3 

2  to  3 

The  last  column  is  the  sum  of  the  products  of  the  degree  squares  by 
the  inches  at  the  top  of  each  column.  The  area  between  the  i.o  inch 
and  2.0  inch  rainfall  line  is  considered  t6  have  a  rainfall  of  1.5  inches ; 
between  2.0  and  3.0  inches  the  rainfall  is  2.5  inches,  etc. 

The  parts  of  rainfall  passing  St.  Louis  being  taken  according  to  table 
on  page  250,  the  following  results  as  the  quantity  of  water  passing  St. 
Louis  on  successive  days  in  inch-degree  squares  of  water. 


SUCCESSIVE  DAYS.  —  APRIL. 


3 

4 

5 

6 

7 

8 

9 

10 

April  3  .... 

0.7 

0.4 

0.2 

0.2 

O.I 

0.0 

0.8 

0.4 

0.2 

0.7 

0.4 

O.2 

o-5 

0.2 

O.I 

April  4  .... 

0.7 

0.4 

0.2 

i-5 

0.8 

0.4 

1.4 

0.7 

0.4 

0.4 

0.2 

O.I 

O.I 

o.o 

O.O 

April  5  .... 

O.I 

0.0 

0.0 

O.I 

0.0 

0.0 

Sums      .     .     . 

0.7 

"•3 

3-i 

3.6 

2.6 

I.I 

0.2 

0.0 

258  METEOROLOGY. 

The  maximum  number  of  the  series  is  3.6,  and  from  the  table  on  page 
252,  the  rise  corresponding  to  it  for  a  stage  of  19.3  feet,  is  6.8  feet. 
This,  added  to  the  rise  of  0.5,  due  to  the  rises  at  Kansas  City  and 
Dubuque,  gives  the  total  rise  at  St.  Louis  as  7.3.  The  observed  rise 
was  7.5.  Such  a  good  agreement,  however,  is  not  to  be  expected  for 
every  rise. 

For  a  rainfall  of  i.o  inch  in  a  day  over  the  whole  of  the  drainage 
area  between  St.  Louis  and  the  line  of  water  travel  four  days  above  it, 
the  estimated  number  of  inch-degree  squares  of  water  passing  St.  Louis 
on  successive  days  from  the  area,  are:  0.5,  1.5,  2.3,  2.3,  and  i.o,  which 
correspond  to  a  rise  at  St.  Louis  in  four  days  of  8.4  feet  with  the  stage 
10  feet,  and  2.8  with  the  stage  26  feet. 

For  a  rainfall  of  i.o  inch  on  each  of  two  successive  days  over  the 
whole  area,  the  number  of  inch-degree  squares  of  water  passing  St. 
Louis,  would  be:  0.5,  2.0,  3.8,  4.6,  3.3,  and  i.o,  corresponding  to  a  rise 
of  1 6. 6  with  the  stage  10  feet,  and  6.1  with  the  stage  26  feet. 

The  above  method  of  predicting  a  river  rise  based  on  observations  of 
rainfall  gives  very  satisfactory  results,  and  is  a  great  improvement  on 
anything  that  has  heretofore  been  done  in  the  way  of  predicting  river 
stages  based  mainly  on  rainfall  observations.  The  official  United  States 
Weather  Bureau  prediction,  made  according  to  this  method  for  St.  Louis 
on  May  26,  1893,  when  the  stage  was  22.5  feet,  was  that  there  would 
be  a  rise  of  5  feet  in  the  next  four  days  at  St.  Louis,  when  the  water 
would  reach  its  highest.  The  stage  attained  on  May  30,  was  28  feet, 
which  was  the  highest  reached  during  the  rise. 

Mississippi  River.  —  The  country  bordering  the  Mississippi  River, 
below  the  mouth  of  the  Ohio,  is  subject  to  overflows.  The  districts 
liable  to  overflow  at  high  water  are :  The  St.  Francis  bottom,  on  the 
west  side,  from  Cairo  to  Memphis,  area  6000  square  miles ;  the  Yazoo 
bottoms,  on  the  east  side,  from  Memphis  to  Vicksburg,  area  6600  square 
miles ;  the  Tensas  basin,  on  the  west  side,  from  Vicksburg  to  the  Red 
River,  5000  square  miles  ;  the  Atchafalaya  basin,  on  the  west  side,  from 
the  Red  River  to  the  Gulf  of  Mexico,  about  6000  square  miles.  The 
area  of  country  flooded  at  one  time  or  another  is  30,000  square  miles, 
inhabited  by  more  than  one  million  people.  The  area,  which  varies  from 
30  to  50  miles  in  width  from  Cairo  to  New  Orleans,  is  shown  on  the  map. 


RIVER-STAGE  PREDICTIONS.  2 $9 

The  country  from  the  Gulf  to  Cairo  is  protected  by  levees  except  the 
St.  Francis  front.  Great  floods  occurred  in  1828,  1844,  1849,  l85°>  ^58, 
1859,  1863,  1867,  1882,  1884,  1890,  1892,  and  1893.  Flood  time  is  from 
February  to  May.  It  begins  on  rare  occasions  in  December.  It  is 
almost  a  proverb  along  the  Mississippi  that  a  full  river  in  January 
portends  a  flood  in  the  spring.  Floods  in  the  lower  Mississippi  are  due 
mainly  to  the  coincidence  of  freshets  from  the  Missouri,  the  upper 
Mississippi,  and  the  Ohio,  coming  into  the  lower  river  at  the  same 
time.  The  Arkansas  and  Red  rivers  are  the  next  important  in  flood 
production.  From  the  way  the  waters  of  the  lower  Mississippi  are 
assembled,  it  is  possible  to  foresee  the  occurrence  of  high  stages  along 
the  lower  river  a  considerable  time  ahead.  The  river  gauge  at  Cairo  is 
the  key  to  all  the  gauges  along  the  lower  river.  Slight  modifications 
to  predictions  have  to  be  introduced  for  the  stages  of  the  Arkansas,  as 
shown  by  the  gauge  at  Little  Rock,  the  stages  of  the  White  River,  as 
shown  by  the  gauge  at  Jacksonport,  or  Newport,  and  the  Red  River  at 
times,  as  shown  by  the  Shreveport  gauge.  In  great  floods  of  the  Mis- 
sissippi, the  discharge  of  water  is  about  2,000,000  cubic  feet  per  second. 

The  map  shows  the  positions  of  gauges.  The  crest  of  a  high  water 
for  Cairo  can  be  foreseen  7  days  ahead.  The  wave-crest  time  from 
Cairo  to  Vicksburg,  599  miles,  is  7  days  when  the  river  is  within  banks. 
The  wave-crest  time  from  Little  Rock  to  Vicksburg,  374  miles,  is  5 
days.  From  Vicksburg  to  New  Orleans,  the  wave-crest  time  is  4  days. 
The  wave-crest  times  are  very  different  in  different  floods,  being  much 
longer  when  the  river  is  out  of  its  banks  and  the  country  along  the 
stream  is  overflowed.  The  Cairo  stage  is  the  key  to  the  stages  along 
the  lower  river. 

For  a  stage  of  41  feet  at  Cairo,  the  St.  Francis  bottoms  begin  to  be 
overflowed.  The  bottoms  act  as  a  reservoir  from  which  the  water  flows 
slowly,  returning  to  the  Mississippi  River  through  the  St.  Francis 
River.  While  the  time  of  wave-crest  travel  from  Cairo  to  Helena,  dis- 
tance 303  miles,  is  only  4  days,  while  the  river  is  within  banks,  yet 
when  the  St.  Francis  is  overflowed,  the  time  is  on  the  average  12  days, 
when  the  crest  at  Cairo  approximates  50  feet.  In  1893  it  was  15  days. 
Helena,  Ark.,  is  just  below  the  mouth  of  the  St.  Francis  River.  It 
takes  37  days  to  fill  the  swamps  above  Cairo  to  a  depth  of  5  feet. 


260 


METEOROLOGY. 


The  same  retarding  influence  on  the  time  of  wave  crests  from  Cairo 
to  Vicksburg  is  caused  by  the  Yazoo  basin  when  overflowed.  The 
crest  time  from  Cairo  to  Vicksburg  has  been  as  high  as  31  days;  on 
the  average  for  the  river  out  of  its  banks  it  is  17  days.  The  time  is 


AREAS  OF  OVERFLOW. 


Locality. 


St. Francis  Basin,  from 
Commerce,  Mo.  to 
Helena,  Ark. 

Left  Bank,  from  Com- 
merce, Mo.  to  Mem- 
phis, Tenn. 

White  and  Arkansas 
Basins,  from  Hele- 
na, Ark.  to  Arkan- 
sas City,  Ark. 

Yazoo  Basin,  from 
near  Memphis, 
Tenn.  to  Vicksburg 

Macon  Bceuf  and  Ten- 
sas  Basins,  from 
Ark.  City,  Ark.  to 
Red  River. 

Left  Bank,  fromVicks- 
burg,  Miss,  to  Ba- 
ton Rouge,  La. 

Atchafalaya  Basin, 
from  Red  River  to 
Bayou  La  Fourche. 

Pontchartrain  Basin, 
from  Baton  Rouge 
La.  to  the  Gulf  of 
Mexico. 

LaFourche  Basin  from 
Donaldsonville,  La. 
to  the  Gulf  of  Mex- 


Total, 


States. 


Miss. 

Ark. 
111. 
Ky. 
Tenn. 


Ark. 


Tenn. 
Miss. 


Ark. 
La. 


Miss. 
La. 


La. 


La. 


2874 

3216 

65 

125 
426 


956 


66,; 


480 

4475 


6085 


2024 


•    — 


6090 

616 

956 

6648 

4955 

415 

6085 


2024 


29700 


AREAS  BY  STATES. 


llinois, 

Missouri, 

Kentucky, 

1'ennessee, 


Square 


125 

453 


Arkansas, 

Mississippi 

Louisiana, 

Total, 


Squ 


4652 
14695 


29790 


greatly  dependent  on  the  breaks  or  crevasses  there  may  happen  to  be 
along  the  river  above  Vicksburg.  When  the  Tensas  basin  is  over- 
flowed, the  river  at  New  Orleans  may  steadily  rise  for  a  month  after 
the  fall  at  Vicksburg  has  set  in. 


RIVER-STAGE  PREDICTIONS. 


26l 


Were  it  not  for  the  levees  a  great  part  of  the  flood  plain  of  the  lower 
Mississippi  would  be  overflowed  every  year,  preventing  cultivation.  In 
most  cases  of  ordinary  high  water  levees  prevent  the  country  from 
being  flooded.  When  the  water  overtops  the  levees  or  the  wave  wash 
causes  a  break,  it  rapidly  widens  and  may  become  miles  in  width  unless 
the  ends  of  the  break  are  secured  by  driving  piling  or  throwing  in 
stones.  The  effect  of  a  break  is  to  lower  the  water  along  the  river 
immediately  below  it.  The  stages  of  water  along  the  stretches  of 
river  lower  down  where  the  overflow  water  returns  to  the  river  are 
higher  than  when  there  is  no  overflow.  At  all  high  waters  there  are 
more  or  less  breaks,  and  they  modify  the  stages  so  largely  that  it  is 
impossible  to  lay  down  rules  for  stage  prediction  along  the  lower  course 
of  the  river,  that  will  give  accurate  results  in  all  cases.  The  con- 
tinual levee  building  that  is  going  on  changes  the  relative  flood  heights 
along  the  lower  river  somewhat.  There  has  been  such  a  great  increase 
in  the  extent  of  levees  since  1886,  that  the  record  of  rises  previous  can- 
not be  used  to  determine  what  the  rises  will  be  at  the  present  time 
at  the  highest  stages. 

The  following  are,  on  the  average,  the  corresponding  stages  for  Cairo 
and  a  number  of  places  on  the  lower  river  as  long  as  the  river  remains 

within  its  banks. 

COMPARATIVE  STAGES. 


CAIRO. 

MEMPHIS, 
3  DAYS  AFTER. 

HELENA,  ARK., 
4  DAYS  AFTER. 

ARKANSAS  CITY, 
5  DAYS  AFTER. 

GREENVILLE, 
6  DAYS  AFTER. 

VlCKSBURG, 

7  DAYS  AFTER. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

20 

I4.I 

21.  0 

23-9 

25 

I8.3 

25.7 

27.7 

24.2 

25.4 

30 

22.5 

31-5 

32.5 

29.7 

32-4 

35 

26.7 

36.2 

37-5 

34-2 

37-3 

40 

30.7 

40.4 

41-5 

38.7 

41-5 

Helena,  Ark.  —  For  the  highest  stage  reached  at  Cairo,  49  to  52  feet, 
the  rise  at  Helena,  Ark.,  continues  for  twelve  days  after,  and  the  aver- 
age rise  in  the  time  is  4.0  feet  when  the  stage  at  Helena  on  day  of 
Cairo  crest  is  43. 5  feet.  The  successive  daily  rises  at  Helena  from  the 
day  of  Cairo  crest  are  as  follows,  on  the  average  :  — 


Day 
Feet 


ist 


2d 

+  .1 


3d 
+  .2 


4th 
+  .25 


5th 
+  -33 


6th 

+  -6 


7th 
+  1.0 


8th 
+  .6 


+.3 


loth 

+  .2 


nth 

+.1 


1 2th 

+  .05 


262  METEOROLOGY. 

The  average  number  of  days  from  the  day  of  crest  at  Cairo  to  the  day 
of  maximum  rise  at  Helena  is  eight,  and  the  rise  continues  on  the  aver- 
age four  days  after  that.  The  greatest  maximum  daily  rise  in  any  case 
is  1.2  feet. 

For  the  highest  stage  reached  at  Cairo,  46  to  49,  the  rise  at  Helena, 
Ark.,  continues  fourteen  days  after  the  Cairo  crest,  and  the  average 
rise  in  that  time  is  2.6  feet.  The  successive  daily  rises  at  Helena 
from  the  day  of  the  Cairo  crest  are  as  follows,  on  the  average :  — 

Day,  ist  2d  3d  4th  5th  6th  7th  8th  gth  loth  nth  I2th  I3th  I4th 
Feet, +.05  +.1  +  .1  +.1  +  .2  +  .2  +.3  +.3  +.3  +.3  +.2  +.2  +.1  o 

v 

The  greatest  daily  rise  is  0.5  foot,  and  occurs  ten  days  after  the  Cairo 
crest.  The  rise  continues  four  days  after  it. 

For  the  highest  stage  reached  at  Cairo,  45  to  46  feet,  the  rise  at 
Helena,  Ark.,  continues  for  eleven  days  after,  and  in  that  time  rises,  on 
the  average,  2.2  feet. 

Vicksburg,  Miss.  —  For  finding  a  rule  to  determine  the  rise  of  the 
river  at  Vicksburg  there  are  available  the  records  of  stages  of  water 
observed  at  Cairo,  111.,  on  the  Ohio  River,  at  Little  Rock  on  the 
Arkansas  River,  at  Newport,  Ark.,  on  the  White  River,  and  at  Madi- 
son, Ark.,  on  the  St.  Francis  River.  The  wave-crest  time  from  Cairo 
to  Vicksburg,  distance  599  miles,  is  from  seven  to  eight  days ;  from 
Little  Rock  to  Vicksburg,  374  miles,  five  days.  As  yet  records  have 
not  been  kept  long  enough  at  Newport  and  Madison  to  make  the  stages 
of  any  service  in  computing  the  Vicksburg  stage. 

Besides  the  water  passing  Cairo  and  Little  Rock  there  is  also  water 
passing  Vicksburg  from  78,000  square  miles  of  additional  drainage  area 
below  those  places.  The  Cairo  stage  is,  however,  the  main  and  dom- 
inating influence  in  producing  high  stages  of  water  at  Vicksburg. 

With  the  record  of  river  stages  since  1872,  proceeding  in  the  same 
manner  as  for  Cairo,  the  following  rule  is  derived  for  finding  the  rise  in 
the  river  at  Vicksburg  in  terms  of  the  rises  at  Cairo  and  Little  Rock. 

When  the  river  has  been  rising  at  Cairo  for  seven  days,  and  has 
reached  a  crest,  the  rise  at  Vicksburg  in  the  next  seven  days  will  be 
equal  to  the  rise  in  the  preceding  seven  days  at  Cairo  multiplied  by  the 
mean  stage  on  the  day  of  the  crest  and  seven  days  before,  plus  one- 


RIVER-STAGE  PREDICTIONS. 


263 


third  of  the  rise  at  Little  Rock  in  the  five  days  preceding,  multiplied  by 
the  mean  stage  at  Little  Rock  on  the  day  of  the  Cairo  crest  and  five 
days  before,  the  sum  multiplied  by  a  factor  depending  on  the  Vicksburg 
stage,  and  divided  by  the  stage  at  Vicksburg  on  the  day  of  the  Cairo 
crest.  When  there  is  a  fall  at  Little  Rock  its  product  is  to  be  taken 
with  a  minus  sign.  The  following  are  the  factors  :  — 

FACTORS  FOR  VICKSBURG  STAGE  FROM  CAIRO,  ILL. 


VICKSBURG  STAGE. 

FACTOR. 

VICKSBURG  STAGE. 

FACTOR. 

Feet. 

Feet. 

25 

0.76 

35 

0.46 

26 

o-75 

36 

0.42 

27 

o-73 

37 

0-37 

28 

0.72 

38 

0-33 

29 

0.71 

39 

0.29 

30 

0.69 

40 

0.25 

31 

0.65 

4i 

0.21 

32 

0.61 

42 

O.I7 

33 

0.56 

43 

0.13 

34 

0.51 

44 

0.09 

The  following  is  an  example  of  the  application  of  the  rule  :  The  stage 
of  water  at  Cairo  rose  from  31.0  feet  on  April  21,  1885,  to  38.2  on 
April  28.  From  April  23  to  28  the  Arkansas  River  at  Little  Rock  rose 
from  10.6  to  28.6  feet.  The  stage  of  water  at  Vicksburg,  April  28,  was 
35.4  feet.  The  rise  to  be  expected  at  Vicksburg  by  May  5  was  then :  — 
0.46(7.2  x  35  +0.33(18.0  x  20)) 


35 


=4.9. 


The  observed  rise  was  4.6  feet. 

This  rule  only  applies  while  the  river  is  within  its  banks,  and  the 
stage  of  river  at  Cairo  is  not  greater  than  42  feet. 

The  flood  line  at  Vicksburg  is  at  41  feet.  The  high  stage  of  April 
25,  1890,  was  49.1  feet.  When  the  flood  water  coming  out  of  the  St. 
Francis  and  Yazoo  bottoms  coincides  with  a  rising  stage  in  the  main 
Mississippi  River,  the  stage  may  possibly  go  as  high  as  51.7  feet,  as 
occurred  in  the  flood  of  1862. 


264 


METEOROLOGY. 


The  rise  of  the  Arkansas  River  at  Little  Rock,  Ark.,  can  be  antici- 
pated two  days  ahead  from  the  rises  at  Fort  Smith,  194  miles  above  it. 
The  corresponding  crests  are  as  follows :  — 

COMPARATIVE  STAGES. 


FORT  SMITH. 

LITTLE  ROCK, 
2  DAYS  LATER. 

FORT  SMITH. 

LITTLE  ROCK, 
2  DAYS  LATER. 

Feet. 

Feet. 

Feet. 

Feet. 

15 

177  ±  1.2 

22 

24.7  ±  0.7 

17 

20.2 

23 
24 

24 

18 

19 

21.0 
22.0 

26 

33 

20 

22.8 

27 

28.7 

21 

23.8 

28 

29.6 

The  flood  line  at  Little  Rock  is  at  23  feet.  The  highest  water,  31 
feet,  occurred  in  1844. 

The  corresponding  wave-crest  stages  for  Vicksburg,  Baton  Rouge,  and 
New  Orleans  are  as  follows : — 

COMPARATIVE  STAGES. 


HIGH-WATER  CRESTS. 


BATON  ROUGE,  3  DAYS  AFTER. 

NEW  ORLEANS,  4  DAYS  AFTER. 

Feet. 

Feet. 

Feet. 

20 

12.6 

6-3 

25 

14.8 

7.6 

30 

19.6 

9-5 

35 

25.6 

"•5 

40 

28.4 

13*1 

45 

32.6 

14-5 

49 

36.0 

15.7 

While  the  river  is  within  its  banks  the  rise  at  New  Orleans  in  four 
days  is,  on  the  average,  one-fourth  of  the  preceding  four-day  rise  at 
Vicksburg. 

When  the  Tensas  basin  is  overflowed  from  breaks  in  the  vicinity  of 
the  Arkansas  River,  the  water  subsequently  returning  to  the  river  lower 


RIVER-STAGE  PREDICTIONS.  26$ 

down,  through  Bayou  Macon  and  the  Red  River,  may  cause  a  rise  to 
continue  in  the  lower  river  long  after  a  fall  has  set  in  at  Vicksburg ;  in 
such  a  case  the  river  at  New  Orleans  may  go  as  high  as  17.9  feet. 

When  the  lower  Mississippi  River  is  very  high,  near  the  tops  of  the 
levees,  rules  for  estimating  the  rises  are  almost  useless,  on  account  of 
the  disturbances  of  the  regimen  due  to  the  water  escaping  from  the 
main  channel  through  crevasses  and  returning  again  to  the  river  at  a 
lower  level.  The  best  method  of  estimating  rises  when  the  river  is 
high  is  to  compare  what  has  occurred  along  the  upper  course  in  recent 
great  rises,  when  the  circumstances  as  to  number  of  crevasses  and 
regions  overflowed  were  somewhat  similar.  Accordingly  the  records 
of  stages  for  the  rises  since  1886  are  given  in  the  following  tables.  In 
the  1891  rise  there  were  no  breaks  in  the  levees,  and  no  areas  were 
flooded.  In  1890  the  Yazoo  and  Tensas  basins  were  flooded.  In  1892 
and  1893  the  Tensas  basin  was  flooded. 


266 


METEOROLOGY. 


COMPARATIVE  RIVER  STAGES. 


DATE. 

CAIRO. 

MEMPHIS. 

HELENA. 

ARKANSAS 
CITY. 

GREEN- 
VILLE. 

VlCKS- 
BURG. 

NEW 
ORLEANS. 

LITTLE 
ROCK. 

1886. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

AprU      4 

44.2 

5 

45-2 

6 

46.0 

7 

46.6 

8 

47-2 

9 

47-8 

41.2 

10 

48.3 

41.8 

ii 

48.8 

42.1 

12 

49-3 

42.4 

13 

49.7 

42.6 

H 

50.0 

43-o 

11 

50.2 
50.4 

34-0 

43-2 
43-4 

11 

50.7 
50-9 

34-0 
34-1 

43-6 
43-8 

19 

51.0  c 

344 

44.0 

434 

38.7 

41.2 

16.1 

20 

50.8 

34-6 

44.2 

43-8 

39-o 

41.4 

15.0 

21 

5°-5 

34-6 

44-4 

44.1 

39.2 

41.6 

16.1 

22 

50.0 

34-8 

44-6 

444 

394 

41.9 

16.1 

23 

49-3 

34-8 

44.9 

44.6 

39-6 

42.1 

14.7 

24 

48.2 

34-8 

45.2 

44-9 

39-8 

42.2 

134 

11 

46.6 

44-5 
41.6 

34-7 
34.7 
34-7 

45-9 
47.1 
47-8 

45-2 
454 
45-6 

40.0 
40.2 
40.3 

42.4 
42.8 
43-° 

12.2 
10.9 
IO.2 

34-8  c 

47-9 

46.0 

40.5 

43-3 

10.4 

29 

34-7 

48.0 

46.3 

40.7 

43-5 

10.6 

30 

48.1  c 

46.4 

40.8 

43-7 

10.6 

May       i 

48.0 

46.4 

40.8 

43-7 

2 

47.6 

46.6 

40.8 

43-8 

3 

47.6 

46.6 

41.0 

43-9 

4 

46.1 

46.8 

41.1 

44.0 

5 

45-° 

46.9  c 

41.1 

44.0 

13.3 

6 

43-8 

46-85 

41.0 

44.1 

13.3 

7 

44.25 

134 

8 

44.25 

134 

9 

44.25 

134 

10 

134 

ii 

134 

12 

134 

13 

134 

H 

'3-5 

1887. 

Feb.     22 

45-9 

23 

46.0 

24 

46.2 

46.4 

26 

46.7 

11 

46.9 
47.0 

Mar.       i 

47.1 

34-8 

434 

2 

47.0 

34-8 

43-5 

RI VER-STA  GE  PREDICTIONS. 


267 


COMPARATIVE  RIVER  STAGES.  —  Continued. 


DATE. 

CAIRO. 

MEMPHIS. 

HELENA. 

ARKANSAS 
CITY. 

GREEN- 
VILLE. 

VlCKS- 
BURG. 

NEW 
ORLEANS. 

LITTLE 
ROCK. 

1887. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Mar.      3 

46.9 

34-9 

43-6 

4 

47.0 

34-8 

43.6 

1        5 

47.0 

34-9 

43-7 

6 

47-3 

35-0 

43-9 

7 

47-8 

35-2 

44-3 

8 

48.3 

35-2 

44.4 

39-6 

43-i 

9 

48.5 

35-3 

44-5 

44.8 

39-7 

43-2 

12.5 

10 

48.5 

35-3 

44-7 

45-0 

39-8 

43-3 

II.6 

ii 

48.3 

35-2 

44-9 

45-2 

39.9 

43-5 

10.7 

12 

47-9 

35-2 

45-i 

45-4 

40.0 

43-6 

IO.O 

13 

47-4 

35-2 

45-3 

45-5 

40.1 

43-7 

9.6 

14 

46.8 

35-2 

45-5 

45-  6 

40.2 

43-8 

9-3 

\\ 

46.3 
45-9 

35-2 
35-2 

45-6 
45-8 

45-8 
45-8 

40.3 
40.4 

43-9 
44.0 

8.9 
8.4 

17 

45-4 

35-2 

45-9 

46.0 

40.5 

44.0 

7-9 

18 

44.9 

35-2 

46.0 

46.2 

40.5 

44.1 

7-4 

19 

46.1 

46.2 

40.6 

44-2 

6.9 

20 

46.2 

46.5 

40.7 

44-4 

6.8 

21 

46-3 

46-5 

40.8 

44.4 

6-5 

22 

46.3 

46.6 

40.8 

44-5 

6.0 

23 

46.2 

46.6 

44.6 

13.6 

5-9 

24 

46.6 

44.6 

13-6 

5-7 

2! 

46.6 

44.6 
44-7 

I3.6 
13-8 

27 

13.8 

28 

13-9 

29 

14.0 

30 

14.0 

3i 

14.0 

1888. 

Mar.     28 

37-6 

29 

40.8 

30 
31 

43-8 

April      I 

44-5 

2 

45-° 

3 

4 

45-2 
45-2 

32.7 

40.4 

38-8 

34-4 

35-6 
36.6 

1:1 

5 

45-2 

33-o 

40.8 

39-3 

34-9 

37-3 

l's 

6 
7 

45-2 
44.8 

33-3 
33-3 

41-3 
41.4 

39-8 
40.3 

35-3 
35-7 

37-9 
38.5 

8.1 
7-7 

8 

44-2 

33-6 

41.8 

40.7 

36-1 

39-0 

7-4 

9 

434 

34-0 

42.2 

41.1 

36.4 

39-4 

7-3 

10 

42.6 

34-1 

42-4 

41.6 

36-8 

39-8 

12.4 

7-7 

ii 

41.6 

34-2 

42.5 

42.0 

37-2 

40.2 

12.5 

12.2 

12 

40.5 

34-2 

42.6 

42.4 

37-6 

40.6 

12.6 

14.8 

13 

34-0 

42.7 

42.8 

38.0 

12.7 

14 

33-8 

42.8 

43-3 

38.4 

12.9 

15 

42.8 

43-8 

38.8 

12.9 

16 

42.7 

44.2 

39-2 

13.0 

17 

42.6 

44.6 

39-5 

I3.I 

268 


METEOROLOGY. 


COMPARATIVE  RIVER  STAGES.  —  Continued. 


DATE. 

CAIRO. 

MEMPHIS. 

HELENA. 

ARKANSAS 
CITY. 

GREEN- 
VILLE. 

VlCKS- 
BURG. 

NEW 
ORLEANS. 

LITTLE 
ROCK. 

1888. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

April    1  8 

44.8 

39-9 

13.2 

19 

45-2 

40.2 

13-3 

20 
21 

45-3 
45-3 

40.4 
40.5 

13-5 
13-6 

22 

454 

40.6 

13.7 

23 

13-6 

24 

13-8 

11 

13.9 
14.4 

27 

14.1 

1890. 

Mar.       i 

424 

2 

43-6 

3 

44-6 

324 

4 
5 

45-5 
46-3 

32.9 
33-2 

42.4 

46.8 

41.7 

464 

16.2 

6 

46.9 

33-6 

42.6 

47.0 

41.8 

14.6 

7 

474 

33-8 

42.8 

474 

42.1 

46.6 

13.2 

8 

47-7 

34-2 

43.0 

47-5 

42.4 

46.7 

12.0 

9 

47-9 

34-3 

43-2 

47.6 

42.5 

46.9 

1  1.0 

10 

48.0 

34-5 

43-3- 

47.6 

42.5 

47.0 

10.2 

ii 

12 

48-3 
48.80 

34-8 

43-5 
43-8 

47.6 
48.0 

42.6 
42.8 

47.1 
47.2 

9-6 

IO.I 

13 

48.6 

35-2 

44.0 

48.1 

43-0 

47-5 

22.1 

'4 

\l 

48.3 
48.2 
48.2 

354 
35-5 
35-6 

44.1 
44.2 
44-3 

48.2 
48.4 
48.6 

43-o 
43-3 

48.0 
47-9 

22.3 
21.9 
21.4 

II 

48.3 
48.2 

35-6 
35-5 

444 
44-5 

48.8 
48.8 

434 
434 

47.6 
47-5 

20.1 

18.2 

19 

48.0 

35-5 

44-7 

48.8 

474 

16.0 

20 

47-5 

35-5 

44-9 

48.9 

43-  ! 

47.2 

21 

47.0 

35-5 

45-1 

49.0 

43-i 

47.1 

22 

46.8 

35-5 

454 

49.0 

43-2 

47.0 

23 

46.7 

35.6 

45-8 

49.0 

43-2 

46.9 

24 

46.6 

35-6 

46.2 

49.1 

43-2 

46.9 

1 

46.6 
46.7 

35-6 
35-5 

46.7 
47-2 

49.2 
494 

43-3 
434 

46.8 
46.8 

27 

46.8 

354 

47-5 

49-5 

434 

46.7 

28 

47.1 

35-3 

47-7 

49-3 

43-3 

46.6 

29 

474 

35-3 

47-7 

49.0 

43-o 

46.5 

47-8 

35-2 

47-7 

48.8 

42.9 

46.4 

31 

48.2 

35-3 

47-6 

48.7 

42.8 

46.4 

April      I 

48.5 

354 

47.6 

48.7 

42.8 

464 

2 

3 

4 

48.5 
48.6 
48.60 

354 
35-5 
35-6 

47-5 
47-5 
474 

48.6 
48.6 

42.7 
42.8 
42.6 

46.4 
46.6 
46.7 

19.9 

5 

48.6 

35-6 

47-3 

48.0 

42.2 

46.8 

20.2 

6 

48.6 

35-5 

47.2 

48.0 

42.2 

47.0 

20.6 

7 

48.3 

35-5 

47.1 

48.0 

42.2 

47.1 

20.  6 

8 

47-7 

35-5 

47.1 

48.0 

42.2 

47-3 

19.7 

9 

46.8 

35-5 

47.1 

48.0 

42.2 

474 

18.3 

RIVER-STAGE  PREDICTIONS. 


269 


COMPARATIVE  RIVER  STAGES.  —  Continued. 


DATE. 

CAIRO. 

MEMPHIS. 

HELENA. 

ARKANSAS 
CITY. 

GREEN- 
VILLE. 

VlCKS- 
BURG. 

NEW 
ORLEANS. 

LITTLE 
ROCK. 

1890. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

April    10 

45.8 

35-5 

47.0 

48.0 

42.1 

47.6 

16.4 

II 

35-4 

47.0 

47-9 

47-7 

I4.6 

12 

35-3 

47.1 

47-9 

47-9 

I3.I 

13 

35-2 

47.1 

47.8 

48.0 

12.0 

14 

47-2 

48.1 

II.  2 

47-4 

48.2 

10.8 

16 

474 

48.3 

I2.O 

17 
18 

47-4 
47-3 

48-5 
48.6 

15-1 

18.1 

21.7 

19 

47.0 

48.6 

15.2 

22.3 

20 

46.6 

48.7 

15.2 

21.6 

21 

45-9 

48.7 

15.40 

20.6 

22 

45-2 

49.0 

15.2 

19.5 

23 

49.0  c 

15.0 

24 

I89I.1 

Feb.     26 

44-7 

27 

45-i 

41.9 

28 

45-5 

42.0 

Mar.      i 

45-8 

33-o 

42.0 

2 

46.0 

33-3 

42.2 

3 

46.1 

33-4 

42.2 

4 

46.2 

33-5 

42.4 

44.4 

44.4 

104 

5 

46.2 

33-6 

42.5 

44-7 

44-5 

9.7 

6 

46.2 

34-0 

42.7 

44-9 

44.8 

9.1 

7 

46.1 

34-2 

42.9 

45-2 

45-5 

9.8 

8 

45-9 

34-5 

43-i 

45-4 

45-7 

ii.  6 

9 

45-5 

34-8 

43-3 

45-6 

45-9 

12.5 

10 

45-3 

34-9 

43-4 

45-8 

40.7 

46.1 

12.7 

ii 

45-° 

34-8 

43-4 

46.0 

41.0 

46.3 

12.0 

12 
13 

44.8 
44-7 

34-7 

43-6 
43-7 

46.2 
46.5 

41.2 
41.4 

46.5 
46.8 

II.  I 

10.5 

H 

44-7 

43-8 

46.6 

41.6 

47.0 

1  0.0 

15 

44.6 

43-8 

46.7 

47-2 

10.  1 

16 

44-5 

43-9 

46.8 

44.9 

47-4 

10.3 

17 

44-5 

44.0 

46.9 

42.0 

47-6 

10.4 

18 

44-5 

44.0 

47.0 

42.0 

47-6 

10.2 

19 

44-5 

44.0 

47.1 

42.1 

47-7 

9.9 

20 

44-5 

44.1 

47-2 

42.2 

47-7 

9-5 

21 

44-4 

44-2 

47-3 

42.4 

47-8 

9.2 

22 

44.2 

44.4 

474 

47.8 

9-i 

23 

44.1 

44-4 

47-5 

42.6 

47-7 

9.2 

24 

44.0 

44-5 

47-5 

42.6 

47-7 

9.2 

25 

43-6 

44.6 

47.6 

42.7 

47-8 

9.0 

26 

43-3 

.  44-7 

47-7 

42.8 

47-8 

8.8 

2 

42.9 
42.5 

44-7 
44.6 

47-8 
47-8 

42.9 
43-° 

47.8 
47-8 

8.6 
8.5 

29 

42.0 

44.6 

47-9 

47.8 

9.0 

30 
31 

41.9 
42.2 

44.6 
44.6 

47-9 
48.0 

43-1 
43-i 

47-9 
48.0 

'5-7 
15-7 

10.3 
"•3 

1  No  breaks  in  levees  this  year. 


2/0 


METEOROLOGY. 


COMPARATIVE  RIVER  STAGES.  —  Continued. 


DATE. 

CAIRO. 

MEMPHIS. 

HELENA. 

ARKANSAS 
CITY. 

GREEN- 
VILLE. 

VlCKS- 
BURG. 

NEW 
ORLEANS. 

LITTLE 
ROCK. 

1891. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

April      i 

42.8 

44-6 

48.0 

43-2 

48.0 

15.7 

12.8 

2 

3 

434 
44.0 

44-5 
444 

48.1 
48.1 

43-2 
43-2 

48.1 
48.1 

I5.8 
I5.6 

'5-5 
16.2 

4 

444 

33-5 

444 

48.2 

48.1 

15.3 

5 

44-7 

33-3 

44.2 

48.0 

15.6 

14.0 

6 

44.8 

334 

44.1 

48.0 

I5.6 

12.6 

1 

44-7 
44-5 

33-5 
33-6 

44.0 
44.0 

48.0 
48.0 

15.5 

'5-5 

11.3 

10.2 

9 

44-3 

33-6 

44.0 

48.0 

15.5 

10 

43-9 

33.8 

44.0 

15.6 

ii 

43-5 

33-9 

44.0 

15-5 

12 

43-3 

33-9 

15.4 

13 

43-3 

33-9 

15.5 

14 

434 

33-9 

154 

1892. 

April      4 

36.2 

5 

38.0 

6 

39-8 

I 

41.7 
43-2 

9 

444 

10 

ii 

45-8 

31-7 

40.3 

39-3 

134 

19.6 

12 

45-8 

32.2 

40.7 

39-9 

13-5 

I7.I 

13 

45-6 

32.7 

41.1 

40-5 

13-5 

154 

14 

45.5 

33-2 

41.5 

41.2 

13.8 

134 

'5 

45.3 

33-5 

42.0 

41.9 

14.0 

I2.4 

10 

45-2 

33-5 

42.2 

42.4 

14.2 

II.2 

'7 

10-3 

18 

44.8 

33-7 

42.6 

434 

14.7 

94 

19 

44-3 

33.8 

42.7 

44-9 

394 

43.8 

14.9 

20 

43-9 

33-7 

42.9 

39-8 

44.1 

14.9 

1:J 

21 

44-3 

34-2 

43-3 

45-6 

40.2 

44.5 

14.9 

1  0.0 

22 

45-3 

34-3 

43-3 

46.0 

40.5 

44.8 

15-5 

10.7 

23 

46.3 

34-2 

434 

46.3 

40.8 

45-0 

154 

n.8 

24 

47.0 

11.9 

3 

34-2 
34.1 

43-5 
43-6 

46.9 
47.1 

4L5 
41.8 

45-8 
46.2 

15.9 
15.8 

I2.O 
12.3 

27 

48^ 

34-2 

43-6 

47-2 

42.0 

46.4 

15.8 

12.0 

28 

48.2 

34-2 

43-7 

474 

42.2 

46.6 

1  6.0 

I  1.0 

29 

48.2 

344 

43-9 

47.6 

42.5 

47.0 

16.0 

IO.I 

30 

47-9 

34-5 

44.0 

47-7 

42.6 

47-2 

1  6.0 

9-5 

May       i 

474 

10.3 

2 

46.8 

34-6 

44-3 

48.0 

42.9 

47-5 

16.1 

12.4 

3 
4 

46.0 
44-7 

34-5 
34-6 

44-5 
44.6 

48.0 
48.1 

43-0 
43-2 

47.6 
47-5 

16.2 
16-3 

14.1 
13.6 

5 

43-o 

344 

44.8 

48.2 

43-3 

47.8 

16.4 

13.0 

6 

40.9 

344 

44-8 

48.4 

434 

47-9 

16.4 

II.O 

7 

38.7 

344 

48.5 

43-6 

48.0 

16.5 

94 

8 

37«7 

43-7 

10.4 

9 

36.7 

33-9 

454 

48.7 

43-8 

48.2 

16.6 

17.4 

RIVER-STAGE  PREDICTIONS. 


271 


COMPARATIVE  RIVER  STAGES.  —  Continued. 


DATE. 

CAIRO. 

MEMPHIS. 

HELENA. 

ARKANSAS 
CITY. 

GREEN- 
VILLE. 

VlCKS- 
BURG. 

NEW 
ORLEANS. 

LITTLE 
ROCK. 

I892. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

May     10 

36.5 

33.6 

45-7 

48.7 

43-8 

48.4 

16.8 

17-5 

ii 

36.6 

33-o 

45-8 

48.8 

43-8 

48.4 

16.7 

20.2 

12 

36.8 

32.5 

45-7 

48.9 

43-9 

48.3 

16.8 

21.8 

'3 

37-4 

31-8 

45-5 

49.1 

44.0 

48.3 

1  6.8 

22.0 

38.4 

3M 

45-2 

49.2 

44.1 

48.4 

16.7 

21.5 

15 

48.4 

16.7 

21.2 

16 

41.2 

31.2 

44-7 

49.1 

43-9 

48.4 

16.7 

24.3 

J7 

41.8 

31.4 

44-9 

49.1 

43-9 

48.3 

16.7 

25.6 

18 

42.5 

3i.7 

44-5 

43-9 

484 

16.9 

26.4 

19 

43-o 

32.0 

44-5 

49.1 

43-9 

48.3 

1  6.8 

27.3 

20 

43-6 

32.2 

44.4 

49-2 

44.0 

48.2 

16.9 

27.9 

21 

44.2 

32.5 

44.4 

49-3 

44.0 

17.0 

27.8 

22 

45  >° 

32.7 

17.0 

27.5 

23 

45  -6 

32.9 

44-4 

494 

44.1 

48.2 

17.1 

27.1 

24 

45-9 

44-3 

494 

44.1 

48.2 

17.1 

26.6 

25 

46.0 

33-3 

44-3 

49-5 

44.2 

48.2 

16.9 

25.9 

26 

46.0 

33-5 

44-3 

49.6 

44.1 

48.2 

16.8 

25.0 

27 

45-9 

33-6 

44-3 

49.6 

44.2 

48.3 

16.9 

24.2 

28 

45-  6 

33-8 

44.4 

49-7 

44.2 

48.2 

16.9 

23-5 

29 

45-5 

34-0 

48.2 

16.9 

22.6 

30 

45-4 

34-1 

49.8 

444 

48.3 

16.9 

21.2 

31 

45  -° 

34-2 

49-8 

444 

48.3 

17.0 

2O.O 

June       i 

44.6 

34-3 

44-7 

49-91 

44-5  l 

48.3 

17.1 

19.8 

2 

44-2 

34-4 

44-8 

49.8 

444 

48.4 

17.2 

21.2 

3 

43.8 

34-4 

44-7 

49-8 

44-3 

484 

17.2 

23.6 

4 

43-7 

34-2 

44.6 

49.6 

44.2 

48.4 

17.3 

24-9 

43-7 

34-2 

484 

17-3 

25.6 

6 

43-9 

34-1 

44.6 

49-5 

44.0 

48-3 

17.3 

25.8 

7 

44.0 

34-0 

44.6 

494 

43-9 

48.2 

17.2 

257 

8 

44.0 

33-9 

44-7 

43-9 

48.2 

17.4 

25-5 

9 

43-9 

33-9 

494 

43-9 

48.1 

17.4 

25.0 

10 

43-7 

33-9 

44-8 

494 

43-9 

48.1 

17.4 

ii 

43-5 

33-8 

44-8 

494 

43-9 

48.0 

17.6 

12 

43-1 

33-8 

48.0 

'7-5 

13 

42.8 

33-8 

45  -° 

494 

43-9 

48.0 

17-5 

42.5 

33-7 

45-0 

494 

43-9 

'7-5 

15 

42.2 

33-5 

45-  * 

494 

43-9 

47-9 

16.9 

16 

41.8 

33-5 

45-4 

494 

43-8 

47-9 

16.9 

17 

41-3 

45-1 

49-3 

43-8 

47-9 

16.8 

IS 

40.5 

49.2 

43-7 

47-9 

16.8 

1893. 

April    1  7 

40.6 

26.4 

32.2 

30.0 

10.3 

18 

41.8 

27.9 

34-6 

30-2 

10.0 

19 

42.5 

28.9 

36.2 

31-7 

20 

43-0 

29.9 

334 

10.0 

21 

30.8 

384 

34-7 

1  0.0 

1  Arkansas  City  would  have  gone  to  51  and  Greenville  to  46  had  not  the  levees  broken, 
A  great  part  of  the  high  water  of  Arkansas  and  White  rivers  never  reached  the  Mississippi,  but 
escaped  over  banks  and  around  the  head  of  Arkansas  levees  in  Amos  Bayou  ridge.  —  WILLIAM 
STARLING. 


2/2 


METEOROLOGY. 

COMPARATIVE  RIVER  STAGES.  —  Concluded. 


DATE. 

CAIRO. 

MEMPHIS. 

HELENA. 

ARKANSAS 
CITY. 

GREEN- 
VILLE. 

VlCKS- 
BURG. 

NEW 
ORLEANS. 

LITTLE 
ROCK. 

1893. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

Feet. 

April    22 

43.5 

31-3 

39-2 

35-8 

10.2 

23 

24 

43-7 

32.0 

40-3 

37-6 

11 

43-2 
42.6 

32-3 

40.6 
41.0 

38.3 
38.8 

"•5 

n.6 

8.7 
12.3 

27 

42.1 

32.8 

41-3 

394 

n.8 

13-6 

28 

41.8 

32.8 

41.6 

39-9 

11.9 

13.5 

29 

41.9 

32.8 

41.7 

40.4 

11.9 

IS-' 

3° 

May       i 

44-6 

33-1 

42.0 

41.4 

12.4 

234 

2 

45.9 

33-2 

42.1 

38.21 

41.8 

12.6 

25.1 

3 

46.8 

33-3 

42.2 

44.81 

38.8 

42.4 

12.8 

25.2 

4 

47.6 

334 

42-3 

454 

43-0 

12.8 

25.1 

5 

48.2 

33-6 

42.4 

46.0 

40.0 

434 

13.8 

24.9 

6 

48.8 

33-8 

42-5 

46.6 

40.7 

44.0 

13.7 

24.0 

7 

49.1 

34-0 

42.7 

44.6 

22.4 

8 

49-2 

34-3 

48.0 

424 

45-2 

14.1 

21.4 

9 

49-3 

34.5 

48.4 

42.9 

45.8 

14.2 

20.9 

10 

49-3 

34-7 

43-3 

48.7 

434 

46.2 

14-3 

20.8 

ii 

49-3 

34-9 

43-5 

48.9 

43-6 

46.8 

14.6 

21.  1 

12 

49-3 

35-o 

43-7 

49.1 

43-7 

47.1 

14.8 

2O.6 

13 

49.2 

43-8 

49.2 

43-8 

47-3 

15.0 

19.9 

14 

49.1 

44.0 

15 

49.0 

35-2 

44-2 

49-5 

44.1 

47-8 

r5-5 

J7-5 

1  6 

48.6 

35-2 

444 

49.6 

44.1 

48.0 

16.1 

'7 

47-9 

35.2 

44-6 

49.6 

44.1 

48.1 

J5-7 

14.8 

18 

47.0 

44.9 

49-6 

44.1 

48.2 

J5'7 

13.6 

19 

45-8 

35.1 

45-2 

49.6 

44.1 

48.2 

15.6 

13.0 

20 

44-3 

35-1 

45-7 

49.6 

44.0 

48.2 

15.6 

21 

46.4 

22 

40.0 

34.9 

47.0 

49-6 

44.0 

48.3 

'5-7 

I2.I 

23 

37-9 

34-7 

47-5 

49-7 

44.1 

48.3 

15-9 

11.6 

24 

36.2 

34-6 

47.8 

49.8 

44.2 

48.1 

II.O 

25 

34-9 

34-2 

48.0 

49-9 

44.2 

47-8 

i6x> 

10.4 

26 

34-3 

334 

47-9 

50.0 

44-2 

47-5 

1  6.0 

9.9 

11 

34-6 

324 

47-7 
47-7 

50.0 

44-2 

47.2 

1  6.0 

9-5 

29 

36.7 

31.2 

47-2 

50.3 

44-3 

47.0 

1  6.0 

14.8 

30 

37-9 

31.0 

50.0 

44.0 

47.0 

1  6.0 

16.7 

31 

38.7 

31.0 

46.0 

49-8 

43-9 

46.8 

16.0 

16.7 

June       i 

39-5 

31-6 

46.0 

49.7 

43-7 

46.7 

16.1 

20.6 

2 

40.3 

31.8 

49.6 

43-6 

46.5 

16.1 

21.4 

3 

41.4 

32.1 

45-6 

49-6 

43-5 

46.4 

1  6.2 

21.5 

4 

43-2 

32.5 

45-3 

49.6 

43-5 

46.2 

16.1 

20.8 

6 

43-3 

32.9 

45-2 

49-6 

43-5 

46.2 

16.2 

19.6 

7 

45-5 

49-5 

434 

46.1 

16.4 

1  8.0 

8 

42.7 

33-2 

45-° 

49.5 

434 

46.1 

16.5 

16.5 

9 

42.7 

334 

44-9 

49-3 

43-3 

46.9 

16.5 

15.0 

10 

43-° 

334 

44-8 

49.2 

43-2 

45-9 

16.5 

14.2 

1  May  10.  —  Top  of  levees  east  bank  of  river  are  up  to  47  feet  Greenville  or  52  feet  Arkansas  City 
gauge,  except  a  few  short  stretches,  which  are  one  foot  lower.  Top  of  levees  west  bank  51  feet 
Arkansas  City  or  45.5  Greenville  gauge.  —  WILLIAM  STARLING,  Chief  Engineer. 


RIVER-STAGE  PREDICTIONS.  273 

CREVASSES  IN  1893. 

(1)  May  n,  6  A.M.  —  Break  50  feet,  i^  miles  below  Lakeport,  Ark.      Latitude 

33°  I*'- 

(2)  May  14.  —  Break  75  feet  wide  at  Millport,  Ark.,  15  miles  south  of  Lake- 

port. 

(3)  May  15,  8  P.M.  —  Levee   broke  3  miles  north  of  Grand   Lake,  Ark.,  300 

feet  wide,  8  to  10  deep. 

(4)  May  1 7,  morning.  —  Levee  broke  \  mile  south  of  Grand  Lake.      Crevasse 

on  Arkansas  side,  all  within  1 2  miles,  3000  feet  of  breaks. 

(5)  May  23.  —  Levee  broke  at  Wylies,  4  miles  below  Lake  Providence,  May 

25,  1 200  feet  wide  ;  May  28,  2100  feet  wide. 

(6)  May  29.  —  Amos   Bayou   levee,  below  Arkansas  City,  broke  this  morning 

300  feet  wide  at  noon.  May  30,  Amos  crevasse,  700  feet  wide.  June  i, 
3400  feet  wide.  June  2,  Wylies  crevasse,  4000  feet  wide. 

(7)  June  19  or  18.  —  Break  on  east  side,  47  miles  above  New  Orleans.      Break 

near  Baton  Rouge.  Baton  Rouge  break  closed  June  15.  June  24, 
Rescue  crevasse,  at  10.30  A.M.,  500  feet  wide,  89  miles  above  New 
Orleans.  June  27,  748  feet  wide. 

The  methods  of  prediction  described  for  the  lower  Mississippi  River 
give  very  good  results  in  practice.  As  an  example,  the  official  predic- 
tions of  the  United  States  Weather  Bureau,  made  in  accordance  with 
the  methods,  and  issued  in  the  special  river  bulletin  of  April  21,  1892, 
are  as  follows,  and  will  serve  to  show  the  accuracy  that  may  be  attained 
by  the  method  :  — 

"  The  river  at  Cairo  will  rise  5.5  feet  in  the  next  7  days,  making  the  stage  50 
feet  by  April  28  (the  actual  highest,  April  28,  was  48.2  feet)." 

"At  Helena  the  stage  will  rise  to  47  feet  by  May  10  (the  actual  highest  was 
45.8,  May  n)." 

"At  Arkansas  City  the  stage  will  rise  to  49.5  by  May  15  (the  actual  highest 
was  49.2,  May  14)." 

"  At  Vicksburg  the  stage  will  reach  48  feet  by  May  15  (the  actual  highest, 
48.4,  was  May  15)." 

"  At  New  Orleans  the  stage  will  reach  16.5  feet  about  May  20  (the  actual 
highest,  17.1,  occurred  May  23)." 

Without  levees  a  great  deal  of  country  bordering  the  lower  river 
would  be  overflowed  at  every  high  water,  and  would  be  untenable  by 
population.  Levees  cannot  entirely  prevent  floods  in  the  lower  Mis- 
sissippi valley.  Such  great  volumes  of  water  as  were  poured  from 


2/4  METEOROLOGY. 

the  Ohio  and  the  upper  Mississippi  into  the  lower  river  in  1882  and 
1890,  would  have  gone  over  the  tops  of  the  most  economical  system  of 
levees  that  the  country  could  construct.  Levees  diminish  the  frequency 
of  overflows,  but  cannot  entirely  prevent  them.  Even  when  not  over- 
topped accidental  breaks  are  liable  to  flood  great  areas. 

The  whole  system  of  levees,  originally  built  by  the  states  bordering 
the  river,  and  in  some  cases  by  private  landowners,  are  now  under  the 
control  of  the  General  Government  and  the  management  of  the  Missis- 
sippi River  Commission.  Floods  in  the  future  will  doubtless  be  less 
frequent  and  less  disastrous  than  in  the  past. 


INDEX. 


A. 

Actinometer,  26. 
Air  constituents,  3. 

expansion,  2. 

general  circulation,  8. 

heating,  4. 

properties,  2. 

specific  heat,  2. 

temperature,  52. 

unsteadiness,  53. 
Air  pressure,  I,  28,  64. 

annual  range,  67. 

distribution,  67. 

diurnal  oscillation,  64. 

effect  of  moon,  65. 

January,  68. 

July,  69. 

thunderstorms,  118. 

monthly  range,  67. 

variation  with  latitude,  65. 
Amazon  River,  206. 
Ancient  lakes,  102. 
Anemograph,  44. 
Anemometer,  40. 
Aneroid  barometer,  33. 

compensation  for,  temper- 
ature, 33. 
Anthelion,  135. 
Anti-trade-winds,  9. 
Arctic  fog,  79. 
Atmospheric     electricity,     81, 

122. 

Aureole,  134. 
Aurora,  136. 
Avalanche  winds,  112. 

B. 

Bacteria,  3. 
Ball  lightning,  120. 
Barber,  115. 


Barometer,  28. 
aneroid,  33. 
gravity  correction,  32. 
reduction  for  temperature, 

32. 

reduction  to  sea-level,  32. 

standard,  31. 

suspension,  31. 

tube,  filling,  29. 

vernier,  30. 

Baric  law  of  wind,  144. 
Beaufort  scale,  42. 
Bench  mark,  50. 
Blizzard,  115. 
Blue  sky,  135. 
Bora,  114. 

Brocken  spectre,  133. 
Buran,  114. 


Callina,  80. 
Calms,  9. 

Cape  Town  wind,  1 1 2. 
Chinook,  114. 
Cirro-cumulus,  74. 
Cirro-stratus,  73. 
Cirrus,  73. 
Cloud-bursts,  93. 
Cloud,  classification,  73. 

formation,  72. 

motion,  45. 

shadows,  76. 

variation,  76. 
Climate  constancy,  100. 
Climatic  changes,  102. 
Cold  waves,  189. 
Corona,  133. 
Crevasses,  273. 
Cumulo-cirrus,  74. 
Cumulo-nimbus,  75. 

275 


Cumulus,  75. 
Current  meter,  51. 
Cyclone,  141. 

axis,  163. 

Bengal  Bay,  171. 

causes  of,  174. 

changing  shape,  149. 

cirrus  clouds,  166. 

cloudiness,  147. 

dangerous  half,  169. 

direction  of  motion,  169. 

extent,  147. 

floods,  198. 

list,  170. 

management  of  ship,  173. 

movement,  148. 

paths,  152. 

place  of  origin,  1 68. 

rain,  147. 

signs,  172. 

thunder,  167. 

tropical,  165. 

wave,  1 66. 

West  Indies,  165. 

D. 

Danger  line,  204. 
Desert  winds,  116. 
Dew,  61. 

Dew-point  apparatus,  36. 
Diamond  snow,  96. 
Dispersion,  132. 
Drainage  areas,  inland,  201. 
Dry  periods,  195. 
Dry  regions,  88. 

E. 

Earth,  area,  7. 
dimensions,  5* 
eccentricity  of  orbit,  7. 


2/6 


INDEX. 


Electrical  storms,  122. 
Electricity  of  atmosphere,  81, 

122. 

Electrometer,  48. 
Equatorial  current,  II. 
Evaporation,  70. 
Evaporometer,  47. 

F. 

Flood  combinations,  209. 

crests,  213. 

earthquake,  198. 

ice-dam,  199,  208. 

line,  204. 

reservoirs,  199. 

river  channel  changes,  200. 

snow,  208. 

streams,  204. 
Floods,  198. 
Fog  bow,  133. 
Fog  formation,  78. 
Fohn  wind,  113. 
Forests  and  rivers,  209. 
Freezing  days,  60. 
Frost,  61. 

Frost  predictions,  194. 
Fulgurites,  120. 

G. 

Garuas,  80. 
Glacier  rivers,  208. 
Glaciers,  97. 
Glories,  133. 
Gold-dust  snow,  96. 
Gradient  and  wind,  146. 
Gradients  and  temperature  fall, 

192. 

Gregale,  114. 
Gulf  Stream,  n. 

H. 

Hail,  98. 

Hair  hygrometer,  38. 

Halos,  134. 

Harmattan,  116. 

Haze,  80. 

Heat  from  sun,  6. 

Heat,  sun's,  12. 

Height,  by  barometer,  I. 

Highs  and  high  pressures,  142, 

161,  175,  182. 
Hoar  frost,  53. 
Horse  latitudes,  9. 


Humboldt  current,  13. 

Mock  suns,  134. 

Hurricanes,  165,  171. 

Monsoon,  109. 

Hypsometer,  34. 

Moon  and  weather,  xvii. 

Mountain  winds,  in. 

I. 

Mozambique  current,  13. 

Icebergs,  98. 

Ice-blink,  136. 

K. 

Iceland  fogs,  80. 

Nephoscope,  45. 

Impermeable  ground,  202. 

Nile,  205. 

Indian  summer,  80. 

Nimbus,  75. 

Insolation,  5. 

Northern  lights,  136. 

Instruments,  14. 

Northers,  115. 

Isobars,  141,  150. 

Nor'wester,  115. 

Isobronts,  119. 

Isohyetals,  141. 

0. 

Isotherms,  141. 

Ocean  currents,  10. 

Optical  appearances,  131. 

K. 

Oxygen,  3. 

Khamsin,  117. 

Ozone,  4. 

Kona,  109. 

Kuro  Siwa,  13. 

P. 

Pacific  Ocean  currents,  13. 

L. 

Pamperos,  115. 

Lake  climate,  63. 

Parhelia,  134. 

Lake  rivers,  201. 

Percolation  gauge,  40. 

Land  and  sea  breeze,  107. 

Periodicity  of  weather,  xxi. 

La  Veche,  1  16. 

Piche  evaporometer,  47. 

Leste,  1  1  6. 

Pluviograph,  44. 

Lightning,  1  20. 

Pogonip,  79. 

Lows  and  low  pressure,  141, 

Polar  bands,  73. 

144,  150,  151,  153,  156, 

Predictions,  185,  195. 

J57>  i58»  I59»  I79»i8i, 

Pressure  gradient,  145. 

183,  184. 

Pressure  types,  188. 

Low  temperature  production, 

Psychrometer,  37. 

1  8. 

Purga,  115. 

Luminous  cross,  135. 

Lysimeter,  40. 

R. 

Radiation  fog,  79. 

M. 

Radiometer,  25. 

Mackerel  sky,  73. 

Rainbow,  132. 

Magnetic  elements,  138. 

Rain,  38,  83,  85,  86,  87,  89, 

Magnetic  storms,  137. 

90»  9*i  93»  i°i,  H7- 

Maximum  temperature,  57. 

Rain  and  dew-point,  183. 

Maximum  thermometer,  19. 

Rain  and  spectroscope,  189. 

Meteorology,  I. 

Rain  gauge,  39. 

Meteors,  xix. 

Rain-gauge  exposure,  40. 

Mirage,  132. 

Rainless  regions,  89. 

Minimum  temperature,  59. 

Rainy  days,  93. 

Minimum  thermometer,  20. 

Red  sunsets,  135. 

Mississippi  River,  258,  266. 

Relative  humidity,  35. 

Missouri  River,  flood,  242. 

Rime,  53. 

Mistral,  114. 

Rivers,  201. 

Mock  moons,  134. 

INDEX. 


277 


Rivers  and  forests,  209. 
River  basin,  203. 
River-channel  changes,  200. 
River  discharge,  215. 
River  gauge,  49. 
River  records,  210. 
River  rise,  212. 
River  slope,  203. 
River-stage    predictions,    21 1, 

2l8,  220. 

Arkansas  City,  Ark.,  261. 

Baton  Rouge,  264. 

Cairo,  111.,  233. 

Carthage,  Tenn.,  238. 

Chattanooga,  Tenn.,  239. 

Cincinnati,  Ohio,  226. 

Davenport,  la.,  241. 

Decatur,  Ala.,  240. 

Eddyville,  Ky.,  238. 

Evansville,  Ind.,  230. 

Florence,  Ala.,  239. 

Greenville,  Miss.,  261. 

Helena,  Ark.,  261. 

Jefferson  City,  Mo.,  241. 

Johnsonville,  Tenn.,  240. 

Kansas  City,  Mo.,  241. 

Little  Rock,  Ark.,  264. 

Louisville,  Ky.,  230. 

Marietta,  Ohio,  225. 

Memphis,  Tenn.,  261. 

Mount  Vernon,  Ind.,  233. 

Nashville,  Tenn.,  238. 

New  Orleans,  La.,  264. 

Omaha,  Neb.,  241. 

Paris,  France,  221. 

Parkersburg,  W.Va.,  225. 

Pittsburg,  Pa.,  222. 

St.  Louis,  Mo.,  243. 

Wheeling,  W.Va.,  225. 

Vicksburg,  Miss.,  262. 
Rivers,  tropical,  205. 
Rivers,  velocity,  213. 

S. 

Saturation,  35. 
Scirocco,  115. 


Secondary  depression,  184. 
Secondary  rainbow,  133. 
Seiches,  198. 
Self-register,  43. 
Self-registering  instruments,  44. 
Simoon,  117. 
Sleet,  98. 
Snow,  39,  95. 
Snow-banners,  136. 
Snow  conductivity,  177. 
Snow  gauge,  39. 
Snow  line,  96. 
Southerly  buster,  115. 
Spectroscope  and  rain,  187. 
Squall,  119. 

Storm  wind,  43,  166,  181. 
Strato-cirrus,  74. 
Strato-cumulus,  74. 
Stratus,  75. 
Sun-dogs,  134. 
Sunshine  recorder,  46. 
Sun-spot  period,  xx. 

T. 

Temperature,  63. 

constancy,  IO2. 

daily  mean,  55. 

daily  range,  54. 

earth,  58. 

greatest  fall,  193. 

lake,  58. 

mean  annual,  56. 

mean  monthly,  56. 

maximum,  59. 

minimum,  59. 

ocean,  57. 

range  at  sea,  55. 

river,  58. 

variability,  60. 
Thalweg,  202. 
Thermometer,  15. 

a'  marteau,  20. 

boiling-point,  17. 

bulb,  23. 

calibration,  15. 


Thermometer  correction,  Pog- 
gendorf,  16. 

deep  sea,  23. 

freezing-point,  17. 

magnifying  front,  23. 

maximum,  19. 

minimum,  20. 

scale,  centigrade,  24. 

scale,  Fahrenheit,  15. 

scale,  Reaumur,  24. 

scale,  De  Lisle,  24. 

sensitive,  23. 

shelter,  24. 

solar  radiation,  21. 

standard,  18. 
Thermoscope,  22. 
Thunderstorms,  118. 
Tornado,  127. 
Trade-winds,  8. 
Tramontana,  114. 
Typhoon,  172. 
Twilight,  135. 

V. 

Vapour  pressure,  35. 

daily  change,  71. 

yearly  change,  71. 
Vernier,  30. 

W. 

Weather-flags,  186. 
Weather-maps,  141. 
Weather  periodicity,  xxi. 
Weather  predictions   178. 
Weather  services,  xxii. 
Wind,  105,  155. 

diurnal  range,  106. 

temperate  zone,  9. 

polar,  10. 

pressure,  42. 
Wind  galls,  133. 
Wind  roses,  117. 
Wind  vane,  43. 


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